Air sampling Part 3 - Impaction

Impaction

Impaction is of two types. One based on the principle of centrifugal action and another based on impingement onto solid.

a)         Centrifugal action

Egs: 1. Air Centrifuge

        2. Reuter centrifugal air sampler

b)      Impingement on solids

Egs: 1. Hollaender & Dalla valle sampler

        2. Slit sampler

        3. Sieve sampler

        4. Andersen six stage or multistage sampler

 Air sampling by Centrifugal action

1.         Air Centrifuge

It is a primitive type of air centrifuge defined by Wells in 1993. In this method, particles from air are centrifuged onto culture medium. Inner surface of the instrument is lined with culture medium. Sampled air passes along a tube which rotates rapidly on its long axis and acts as the inlet. Bacteria  containing particles get deposited on the medium during this centrifugation and develop into colonies on incubation.

2.         Reuter centrifugal air sampler

A modern version of air centrifuge is Reuter centrifugal air sampler. This device is portable and battery operated. This resembles a large cylindrical torch with an open-ended drum at one end.  This open end acts as both inlet and outlet. Drum end encloses an impeller blade rotated by battery power.A plastic strip coated with medium is inserted along the inner side of the drum. Air entering the drum is subjected to centrifugation by the rotation of the impeller blade when switched on. Suspended particles in air attach to the medium on plastic strip and can be removed and  incubated after sampling period at required temperature.

Advantages

❖         Simple to use

❖         Convenient for transportation

Limitations

❖         Less efficient than Slit sampler in detecting particles below 5mm diameter

❖         Volume of air sampled can not be accurately measured and controlled

 

Air sampling by Impingement on solids

Impingement on solids is based on the principle that when air is drawn from the environment at a fixed rate and suspended particles are allowed to impinge on the surface of an agar plate, each particle containing viable bacteria will form a colony on incubation.

 1.         Hollaender & Dalla valle sampler

Consists of a brass container with a removable bottom. It is fitted with an inverted glass funnel which will act as the inlet. Inside of the funnel and rim is to be wiped with alcohol before use. At the lower part of the container, a petri dish base with medium is placed and screwed tightly against the funnel. Funnel is kept just above the media, approximately at 2mm above and should not touch the media. Chamber has an outlet that can be connected to a vacuum pump. When the pump is switched on,  and air borne microbes are impinged upon the agar medium. Vacuum pump can be connected in series with a flow meter to measure the exact volume of air  sampled. Effective sampling rate is 28 lit/minute.  It is a simple, efficient and portable air sampler.

 

 2.   Slit sampler (Bourdillon slit sampler)

An efficient and convenient device for enumeration of bacteria carrying particles in the unit volume of air. This method was introduced by Bourdillon et al  in 1941 and hence also called Bourdillon slit sampler. Instrument consists of an enclosed box with an air tight door. It has a narrow slit on the top measuring 0.33 mm width, 27.5 mm length and vertical parallel sides of about 3 mm depth. Slit acts as the inlet. Outlet of the box is connected to a vacuum pump and maintains a negative pressure of 22.6 mm Hg inside. At correct negative pressure, air at a rate of 1 cubic foot/min enters through slit. At the bottom, a rotating platform is present to keep the agar plate. This platform is usually coated with adhesive or gripping material to prevent slipping out of the agar plate during sampling. Slit is exactly 2mm above the plate. As the plate is rotating with the platform, particles are deposited over the entire surface of the plate.

In this method, a sterile,dry agar plate with an even surface is taken. Sampling area, date, time, duration  of sampling, volume of air sampled etc are marked. Slit is made free from dust with alcohol and by inserting the edge of a stiff paper. Petri Plate is placed on the platform through the door and height is adjusted to be 2mm below the slit. The motor that rotates the plate and the vacuum pump are switched on simultaneously. After the sampling period, both are switched off and the plate is taken out carefully, incubated to get the colonies. It can be also used to count spores and hence useful in plant disease forecasting.

Limitations

❖         Chance  of contamination from skin and clothes of the operating person while handling the plate.   

❖         Unnecessary movements should be avoided during sampling.

A modification is Size Grading Slit Sampler with slits of different sizes kept in series. Sets of plates used to collect particles of different sizes. It is highly efficient and found to collect 95% of water droplets sprayed into air. It can even collect respiratory secretion droplet nuclei of 0.2µm diameter.

2.1. Hirst Spore trap

Hirst spore trap is an example of a slit sampler. Was developed by Hirst in 1952. Also known as Burkard Spore Trap. It is a volumetric air sampler that is one of the standard devices for monitoring airborne pollen and spores. Widely used by the plant pathology community.

It is a type of suction slit impactor used for pollen and spore sampling. First sampler of this type was designed  in 1952 byDr James Hirst, a plant pathologist in England. So samplers of this type are usually referred to as Hirst spore traps.

In  Burkard trap, air is drawn into a 14 mm X 2 mm orifice at a rate of 10 lit/ min and any air borne particles with sufficient inertia are impacted on either a greased tap or a greased microscopic slide beneath the orifice. & days continuous sampling is possible with removal of slides occasionally.

3. Sieve sampler

This is a mechanically simpler instrument. More or less similar to the slit sampler.  Particles containing microorganisms are distributed over the plate as separate air jets through several holes in the sieve plate. Perforated sieve is acting as  the inlet and outlet connected to the vacuum pump. After sampling, petridish is removed carefully and incubated to get the colonies.

4. Anderson sampler

Also known as Anderson six stage/eight stage/ multistage sampler. Here several impingers (6 to 8) with holes of different sizes are arranged in a series. This is an indigenous device that selectively trap different  particles  according to their size. It consists of a stack of 6 to 8 metal sections that are fitted together with ring seals to form an airtight cylinder. Each metal section has a perforated base. Number of perforations in each section is the same, but the size of perforations progressively reduces from top of the column to the bottom. Open petri plates with media are placed between each metal section, resting on three studs. When fully assembled, an electric motor sucks air from the bottom of the unit making air to enter through the inlet at the top and passes down through the cylinder. Air sucked in the top of the column travels relatively at a low speed towards the first agr plate and so only the largest particles impact on the first agar plate. The air travels round the edge of the agar plates and through the perforations to the second agar plate and so on. As this continues down the stack, same volume of air is forced to travel through successively smaller perforations, so air speed is progressively increased. Thus even smallest particles can impact onto the lower agar plates. After 5-15 min, metal plates are separated and petri dishes are incubated. Anderson sampler mimics the deposition of air borne particles in the human respiratory tract. 

    

 

 

 

Air sampling techniques- Part 2 - Based on gravitation

 

Air samplers based on Settling under Gravity include Hesse’s tube and Settle plate method.

1.         Hesse’s Tube

 

Hesse’s tube is a horizontal glass tube with a layer of solid media at the bottom. Particles would settle on the media based on gravity as air entered  into the tube through inlet. Larger particles settle faster than the smaller ones. Upon incubation, colonies develop on the medium. If the tube is long enough or the flow is sufficiently low, all particles will settle out before reaching the outlet end.

2.         Settle Plate Method

 In this method, Petri dishes containing sterile agar medium or any suitable medium of known surface area are used.  Media surface should be dry. Bacteria carrying particles are allowed to settle on to the medium based on gravitational force for a given period of time and incubate at required temperature. Count of colonies show the number of viable bacteria containing particles.

Choice of the media depends on the type of organism to be enumerated. Nutrient agar is commonly used to enumerate most of the pathogenic, commensal and saprophytic bacteria. Selective media like Blood agar, Malt extract for molds also can be used for particular pathogens.

First,  the agar plates are labelled with place, date, time of sampling, duration of exposure etc.  Then the plates are kept open in the selected area for the required time period.  Optimal duration of exposure would provide well developed, readily countable, isolated colonies, approximately between 30-100. Number of colonies depends on the dustiness of air. In occupied rooms and in hospitals, duration of sampling should be less as there is a chance for more organisms. Plates should always be kept 1 m above ground level to avoid settling of particles from ground. Immediately after sampling, the petri plates   are closed with a lid and incubated for a required time period. For aerobic bacteria, incubation at 37^o  C for 24 hrs and for saprophytic bacteria incubation at 22^o  C for 3 days are usually done. Fungus require incubation at room temperature for 1-2 weeks.

Advantages

❖      Simple, economical and easy to perform method

Limitations

❖      Larger particles settle easily and hence  total number of bacteria carrying particles per volume cannot be measured

❖      Growth of settled organisms will depend on the type of media used

❖      Air current and temporary disturbances in the sampling area would affect the count. So multiple plates have to be used.

Air sampling techniques- Part I

 

Air sampling is the enumeration of microorganisms present in air using various techniques. It is useful in disease forecasting, especially in plants. Based on the principle, air sampling techniques can be divided as follows. 

I)  Settling under Gravity

Egs: 1. Hesse’s tube  

         2. Settle plate method

II) Impaction

Impaction is of two types. One based on centrifugal action and another one based on impingement onto solid.

a)    Centrifugal action

Egs: 1. Air Centrifuge

        2. Reuter centrifugal air sampler

b)     Impingement on to solids

Egs: 1. Hollaender & Dalla valle sampler

        2. Slit sampler

        3. Sieve sampler

        4. Andersen six stage or multistage sampler

III) Filtration 

Egs: 1. Tube sampler

        2. Millipore filter

IV) Impingement into liquids

Egs: 1. Raised impinger

        2. Bead bubbler device

        3. Lemon sampler

V)  Electrostatic forces

Egs: 1. Litton large volume air sampler

 

 

 

                       

 

Methyl Red – Voges Proskauer (MRVP) tests

 

The methyl red (MR) and the Voges-Proskauer (VP) tests are used to differentiate two major types of facultative anaerobic enteric bacteria that produce large amounts of acid and those that produce the neutral product acetoin as end product.  MRVP tests are of value in the separation of Escherichia coli and Enterobacter aerogenes, both coliform bacteria, which appear virtually identical except for certain physiological differences that are used as indicators of the sanitary quality of water, foods, food-production and eating establishments.  Both these tests are performed simultaneously because they are physiologically related and are performed on the same medium MRVP broth. Opposite results are usually obtained for the MR and VP tests, ie., MR+, VP^_    or MR^_, VP+.

Aim

To find out whether an organism is MR-VP positive or negative.

Principle

The hexose monosaccharide glucose is the major substrate oxidized by all enteric organisms for energy production.  The end products of this process will vary depending on the specific enzymatic pathways present in the bacteria. In MR test the pH indicator methyl red detects the presence of large concentrations of acid end products.  Although all enteric microorganisms ferment glucose with the production of organic acids, this test is of value in the separation of    E. coli and E.  aerogenes.

Both of these organisms initially produce organic acid end products during the early incubation period.  The low acidic pH (4) is stabilized and maintained by E. coli at the end of incubation.  During the later incubation period, E.  aerogenes enzymatically converts these acids to nonacidic end products such as 2,3-butanediol and acetoin (acetyl methyl carbinol), resulting in the elevated pH of approximately 6.  The methyl red indicator in the pH range of 4 will remain red, which is indicative of a positive test.  At a pH of 6, still indicating the presence of acid but with a lower hydrogen ion concentration, the indicator turns yellow and is a negative test (Figure 1).

The VP test determines the capability of some organism to produce non acidic or neutral end products, such as acetyl methyl carbinol, from organic acids that result from glucose metabolism, which is characteristic of E.  aerogenes (Figure 2).  The reagent used in this test, Barritt’s reagent, consists of a mixture of alcoholic α-naphthol and 40% potassium hydroxide solution.  Detection of acetyl methyl carbinol requires this end product to be oxidized to a diacetyl compound.  This reaction will occur in the presence of the α-naphthol catalyst and a guanidine that is present in the peptone of the MRVP medium.  As a result, a pink complex is formed, imparting a rose color to the medium.  Development of a deep rose color in culture 15 minutes following the addition of  Barritt’s reagent is indicative of the presence of  acetyl methyl carbinol and represents a positive result.  The absence of rose coloration is a negative result (Figure 3).

Requirements

24 hr nutrient broth cultures of species Bacillus, Streptococcus, Staphylococcus, Pseudomonas and E.coli.

MR-VP broth (5ml / tube), Methyl red indicator, Barritt’s reagent A and B, Bunsen burner, inoculating loop, glass marking pencil, dropper etc.

Procedure

1.  Divide the total MR-VP tubes into two sets.  Using sterile techniques, inoculate each set of   MR-VP broth with given cultures of organisms.  Uninoculated tubes serve as control.

2.  Incubate the tubes at 37^o C for 24-48 hrs.

3.  Add 5 drops of methyl red indicator to the first set of organisms and observe the color change for MR test.

4.  To the other set of tubes, add 12 drops of Barritt’s reagent A and 2-3 drops of Barritt’s reagent B.  Shake the tubes gently for 30 seconds with the caps off to expose the media to oxygen.

5.  Allow the reaction to complete for 15-30 min and observe the color change for VP test.

Observations

In the MR test, the MR-VP broth remained red color in the tubes inoculated with Bacillus, Streptococcus, Staphylococcus and E.coli. No change observed in tubes inoculated with  Pseudomonas sp.

In the VP test, no development of a crimson to ruby red color formed in any of the tubes inoculated.

 

Result

Among the given culture of organisms Bacillus, Streptococcus, Staphylococcus and E.coli are MP positive and VP negative whereas  Pseudomonas sp. is MR and VP negative.  

       Barritt’s reagent A and B

 Solution A

Alpha-naphthol                                   5.0 g

Absolute ethanol                                 95.0 ml

(Note: Dissolve the alpha- naphthol in the ethanol with constant stirring).

Solution B

Potassium hydroxide                          40.0 g

Creatine                                               0.3 g

Distilled water                                    100 ml

(Note: Dissolve the potassium hydroxide in 75 ml distilled water. The solution will become warm. Allow to cool to room temperature. Add the creatine and stir to dissolve. Add the remaining water. Store in a refrigerator). 

MR-VP broth                                            

Peptone                                               7.0 g

Dextrose                                              5.0 g

Potassium phosphate                           5.0 g

Distilled water                                    1 litre

pH                                                       6.9

International Standard Serial Number (ISSN)

 

ISSN is an 8-digit identifier code assigned for regular publications like newspapers, journals, magazines and periodicals of all kinds including electronic publications.

1.    What is ISSN?

Authors always prefer to publish their articles in periodicals with ISSN. ISSN means International Standard Serial Number which is a unique international identifier for serial publications. It is universally accepted as a means to identify serial publications and are assigned to journals since 1970s.  From the end of  1990s, ISSN is also assigned to electronic serials (online, CD-ROM, DVD etc.) and to electronic “ongoing integrating resources” like websites and databases. Thus ISSN formed an eight-digit number which is an identification for periodical publications, including electronic serials. The ISSN is a numeric code which is used as an identifier. The ISSN number is published as -the acronym ISSN followed by two groups of four digits separated by a hyphen. The eighth character is a control digit calculated according to a modulo eleven algorithm on the basis of this seven preceding digits. This eighth control character may be an “X” if the result of the computing is equal to “10”, in order to avoid any ambiguity (figure 1).

 

Fig 1- ISSN Number

2.    Importance of ISSN

ISSN number and copyright of the journal are not at all related and the procedure to obtain both are entirely different. ISSN is generally printed on a prominent place, such as at the top right hand corner of the front cover, near the title on every issue. The number of copies produced is not related to the ISSN assignment. ISSN is assigned to publications that are intended to be published continuously on a regular or irregular basis. Eg: Journals, Proceedings, Annual reports, Quarterly reports, Biannual reports, Bulletins, Newsletters, proceedings etc. The ISSN identifies the title of the serial publications and the same ISSN can be used as long as the title remains the same. If there is any change in the title, it is mandatory to inform the ISSN office. Title change would also cause change in ISSN. ISSN is also assigned to good quality ‘Online publications”. Also, other types of regular electronic publication formats, such as DVD’s and CD-ROM’s are assigned with ISSN. If same periodical is published in different languages, then separate ISSN is required for each language. There is no expiry date for ISSN. If the publisher is unable to publish the forthcoming 2 issues, then also it should be intimated to the ISSN office.

3.    How to get ISSN?

Each country is having a respective national authority for ISSN. Publishers can apply for ISSN in advance of publication. Application can be completed and submitted online or can be sent in printed format through post or fax to the ISSN international authority for international publication and ISSN national authority. All details can be had from the ISSN website http://www.issn.org. Basic informations required to apply for ISSN includes the proposed title of publication, frequency of publication, proposed start date (month / year) and publisher name and address. However, first issue of publication has to be sent to the centre in order to verify and validate the details. This copy will be passed through to the Legal Deposit Office and subsequent issues can be sent directly to them.

ISSN can be also obtained for existing serial publications. Application for that should include all above mentioned details and some additional informations and documents as mentioned below.

·         For printed publications –A copy of  recent issue

·         For on-line journals- Either the URL or any passwords required (as requested on the form); or a printout of the title screen, and any screens giving publisher information (company name and the place of publication).

·         For CD-ROM or diskette journals- a sample copy or a printout of the title screen together with copies of the labelling, documentation and packaging.

Sample ISSN numbers of the publications

ISSN 0027-9633 (online)

ISSN 0027-9634 (print)

Reference

Achalare, R. A., Patil, S. V., Patil, S. S. 2014. Significance of ISSN and ISBN in Publications, Journal of Current Pharma Research, 5(1), 1378-1381.

 

 

 

 

Air borne bacterial diseases

A number of diseases are transmitted through air. Such diseases are called airborne diseases. Airborne diseases are divided into bacterial, viral and fungal diseases. Microorganisms released as droplets, Droplet nuclei,Infectious dust and spores are easily disseminated through air. Airborne bacterial diseases are mainly respiratory tract infections transmitted by droplet inhalation or by contact with contaminated inanimate objects. Upper respiratory tract consists of nostrils, pharynx and tonsils. Lower respiratory tract consists of larynx, trachea, bronchi, brachiolus, alveoli and lungs. Transmission of fungal diseases is more through air. But the airborne bacterial diseases are more in number. Common airborne bacterial diseases are as follows.

1.        Brucellosis

2.        Meningococcal meningitis

3.        Meningitis by Haemophilus influenzae

4.        Pulmonary Anthrax 

5.        Diseases by Streptococcus

6.        Atypical pneumonia

7.        Psittacosis

8.        Diphtheria

9.        Pertussis (Whooping cough)

10.    Tuberculosis

11.    Legionellosis

12.    Tularemia

 1. Brucellosis

Brucellosis is also known as “undulant fever” because of its characteristic raising and falling nature of temperature.Brucellosis is a zoonotic disease caused by Brucella species. Brucella species includes B. abortus, B. melitensis,B. suis and B. canis. They are tiny, faintly staining Gram Negative coccobacilli. Brucellosis is a disease of humans, animals and birds, mainly caused by  B. suis, commonly occurring in infected swine. Inhalation of aerosol during processing of infected swine causes infection in humans. Infection is commonly found in veterinarians, butchers and slaughterhouse workers. Hence it is considered as an occupational disease. In humans the disease is characterized by prolonged and undulating fever, headache, chill, generalised aches and pains of muscles and joints. Mortality rate is low, less than 2%.

 2. Meningococcal meningitis

Meningitis is the inflammation of meninges. Meninges is the three layered membrane covering the spinal cord and brain. The term meningitis is derived from a Greek word. “Meninx” means  Brain and “itis” means inflammation. This is  considered as a potentially serious disease as the microbial invasion is to the nervous system. Meningitis is divided into bacterial or septic meningitis and aseptic meningitis. Meningococcal meningitisIs a fatal septic meningitis caused by Neisseria meningitidis. Neisseria meningitidis Is a gram-negative diplococci.

About 10% of population are healthy carriers and harbour this bacteria in the back of the nose and throat. Healthy carriers, Convalescent carriers and infected persons release this cocci as respiratory droplets by activities like coughing, sneezing, kissing, talking etc. Disease is also transmitted through sharing of utensils. No animal reservoir is found for this disease. Inhalation of these bioaerosols will lead to the infection

Organism first colonize in the nasopharynx and penetrate the mucosal barrier to reach the blood stream. They cross the blood-brain barrier and enter cerebrospinal fluid (CSF) and cause meningitis.

 3.  Meningitis by H. influenzae

Mode of infection and disease are the same as above. But the infection is commonly found in children between 6 weeks and 2 years of age. The causative agent is Haemophilus influenzae which is a Gram Negative coccobacilli. Infection can also cause secondary pneumonia in patients.

 4. Pulmonary Anthrax

This is also called ‘Wool sorters disease”. Pulmonary anthrax is mainly a disease of herbivorous animals. It is a highly infectious animal disease that can be transmitted to humans by direct contact with infected animals like a cattle,goat, sheep etc or through their products.

Anthrax is of three types. Human infections through cut or abrasion of skin lead to cutaneous anthrax.   If spores reach gastrointestinal tract, it cause gastrointestinal anthrax. Inhalation of the spores leads to pulmonary anthrax or wool sorters disease. Pulmonary anthrax is seen only in human beings. Causative agent is Bacillus anthracis which is large, Gram Positive, aerobic, endospore forming bacteria. It is world wide in its distribution. Disease transmission is by inhalation of the dust contaminated by animal products. Usually through the sorting or combing of raw wool contaminated with spores. Spores are usually 1 to 2 to µm in diameter and  can enter the lower respiratory tract easily and lodge in the alveolar spaces. There they are engulfed by alveolar macrophages. But can resist and survive the phagocytosis, germinate within the endosome. Bacteria then spread to regional lymph nodes and eventually reach the bloodstream. This will result in pulmonary anthrax which is characterized by massive pulmonary edema, hemorrhage and respiratory arrest.

 5.  Diseases by Streptococcus

A number of airborne bacterial diseases are caused by Streptococcus species. Streptococci  are large group of Gram Positive cocci and most significant one causing air borne infections is S.pyogenes. Infection is either due to the multiplication of pathogen or due to the toxin produced by the organism. Infection may range from mild diseases like most  common “Strep throat” to life threatening conditions. Organisms reside in the nose and throat and are transmitted through respiratory droplets.

a) Streptococcal pharyngitis

This disease is commonly called Strep throat or tonsillitis or sore throat. It is a common mild infection predominantly found in children of 5 to 15 years of age. Symptoms include fever, headache, red and sore throat, enlargement of lymph nodes of neck. Often results in bleeding throat.

b) Scarlet fever

Sometimes Streptococcal pharyngitis proceeds to Scarlet fever. This condition is due to the production of “erythrogenic toxin”  by Streptococci. The disease characterized by strawberry tongue and red skin rashes.

c) Rheumatic fever

3% of untreated upper respiratory tract infections by Streptococcus pyogenes and repeated early childhood infections may lead to a condition known as Rheumatic fever. This disease is characterized by inflammation and degeneration of heart valves and joints.

d) Streptococcal pneumonia 

About 60-80 % of all respiratory diseases known as pneumonia are caused by Streptococcus pneumoniae. Infection occurs in those individuals with predisposing factors such as viral infections of respiratory tract like common cold, or physical injury to the respiratory tract, alcoholism, or diabetics. Causative agent Streptococcus pneumoniae is a gram-positive organism, normally found in the upper respiratory tract. Virulence factor of this organism is the capsular polysaccharide composed of hyaluronic acid. Capsular polysaccharide protects the organism from ingestion and killing by phagocytes. Thus bacteria will multiply in alveolar spaces and also produce toxins called “pneumolysin” which may destroy the host cells. Alveoli get filled with blood cells, fluids and become inflamed. Lung inflammation is a characteristic feature of this disease. Sputum is often rust-colored due to the blood coughed up from lungs. Onset of the disease is a abrupt with chills, hard laboured breathing and chest pain. Inhalation of aerosols liberated from the infected person leads to infection.

 6. Atypical pneumonia

 Is an infection of lower respiratory tract  and also called “walking pneumonia”. Causative agent is Mycoplasma pneumoniae. Symptoms are gradual in onset and are milder than typical Pneumonia. Symptoms include  persistent cough, headache, fever, sore throat, chest pain while deep breathing etc. Cough, sneeze etc will liberate bioaerosols from the patient and inhalation of these will lead to infection.

 7. Psittacosis

This infection is mainly acquired from birds. Infection may range between mild respiratory diseases to severe pneumonia. Infection may sometimes lead to encephalitis, coma and convulsions. Sometimes fatal infections may occur. Causative agent is Chlamydia psittaci. Pathogen is liberated as infectious dust from the infected avian faeces. Inhalation of the infectious dust will cause disease in humans. Hence this is considered as an occupational disease of poultry workers and handlers.

 8. Diphtheria

Causative agent is Corynebacterium diphtheriae which is a gram positive bacilli. Is mainly found in children. Toxins produced by these microbes cause generalized toxemia in patients. Infection occurs in the upper part of the respiratory tract and nose, throat and tonsils get inflamed.  Also lymph glands in the neck region swell and result in “bull neck appearance”. Powerful exotoxin produced by the pathogen destroys the cells cof the epithelial lining. Thus  the dead cells, mucus and scavenger cells pile up and form a pseudo membrane. This membrane is leathery in consistency and cause respiratory blockage and death by suffocation, particularly in young children. Tracheotomy (Cutting hole in throat) are performed to save children. Sometimes, toxin diffuses to blood stream, causing widespread damage particularly to the heart.

 9. Pertussis (Whooping cough)

Is highly infectious and potentially lethal disease caused by Bordetella pertussis which is a Gram negative coccobacilli. This disease mainly affects infants and young children below 4 years of age. No non human reservoir for this disease and infected children act as source of infection. From infected person the pathogen is liberated as aerosols through activities like talking, coughing, sneezing, laughing etc. Bacilli binds to ciliated epithelium of upper respiratory tract and secrete toxins. The function of toxin is to clear mucus from air passages and hence they damage cells. Net effect is the build up of stick glue like mucus in airways. This will result in common cold like symptoms followed by spasms of violent, hacking, persistent, recurrent cough with at least 15 to 20 coughs at a time. This is actually an attempt to remove the accumulated mucus from the respiratory tract. But these episodes result in oxygen deficiency, triggering deep and rapid inspirations through the partially obstructed air passages, resulting in characteristic ‘whoop’. Coughing may be so violent that it may cause vomiting,  hemorrhage and even brain damage. It may last for several weeks or months.

 10.  Tuberculosis

Tuberculosis or TB is a severe lower respiratory tract disease. Causative agent is Mycobacterium tuberculosis which is an acid fast bacteria. These bacteria have high mycolic acid content in the cell wall which makes a waxy coating on its surface and organisms are seen as clumps. These are highly virulent bacteria and even a single organism is  sufficient to initiate an infection. In a susceptible host, the inhalation of a single particle carrying a viable tubercle bacillus and  small enough to reach the lungs is capable of producing infection. Disease is characterized by loss of appetite, fatigue, weight loss, night sweats and persistent cough. Inhalation of aerosols liberated from the patient is a mode of infection.

 11. Legionellosis

This is also a lower respiratory tract infection. This disease is considered as a type of bronchopneumonia or atypical pneumonia. Causative agents are coming under Legionella species.  Legionella species Are Gram Negative bacteria found naturally in fresh water bodies. They may contaminate Air conditioners, cooling towers or showers and cause infection. No person to person transmission of disease occurs.  Legionellosis is of two types. Pontiac fever and Legionnaires disease. 

Pontiac fever is a self limiting, non-fatal disease. It is caused mainly by Legionella  pneumophila. Symptoms include fever, chills, dry cough and headache  with mild upper respiratory tract infections resembling acute influenza. This resolves spontaneously and often goes undiagnosed. Inhalation of mist from water sources like air conditioners, cooling towers, showers etc., contaminated with the pathogen is mode of infection.

Legionnaires disease, unlike Pontiac fever, is severe bronchopneumonia and can be fatal. Here also the causative agent is Legionella  pneumophila. Symptoms include fever, chills, dry cough, vomiting and diarrhoea.  Pathogen occurs in natural air and in freshwater. At times they enter and proliferate in cooling towers, air coolers and showers. Praying and splashing of water containing pathogens may produce aerosols which are disseminated in air and inhalation causes infection. 

 12.  Tularemia

Causative agent of the disease is  Francisella tularensis, which is a Gram negative bacilli. This is one of the most virulent bacteria, inhalation of as few as 10 organisms is sufficient to establish the disease. It is also known as rabbit fever as it is a zoonotic disease transmitted from natural reservoirs of small mammals  like rabbits, mice and squirrels. Here also, there is no transmission of disease from person to person. Inhalation of dust particles liberated from the infected animals result in severe respiratory illness leading to pneumonia.