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Let us make an in-depth study of the tubercle bacilli. The below given article will help you to learn about the following things:- 1. Mycobacterium Tuberculosis 2. Pathogenesis and Disease in Man 3. Transmission of Tubercle Bacilli 4. Anonymous Mycobacteria and 5. Application to Nursing.
Mycobacterium Tuberculosis:
Order Actinomycetales, family Mycobacteriaceae, genus Mycobacterium, include bacteria which are characterised by their ability to branch and by their acid-alcohol-fast properties. The genus Mycobacterium consists of organism responsible for tuberculosis and leprosy and number of acid fast saprophytes.
Mycobacterium tuberculosis, causing tuberculosis in man, was discovered by Robert Koch in 1882.
Morphology of Mycobacterium Tuberculosis:
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Myco. tuberculosis (G . mykes — fungus or mould) is a slender, straight, or slightly curved pleomorphic rod, 0.5 –4µ in length and 0.3µ in breadth. These organisms are non-motile and do not form spores or capsules (Fig. 30.1).
Though they are Gram-positive, they cannot be readily stained by Gram method, but they can be easily stained by Ziehl-Neelsen method, as they resist decolourisation with 20-25 per cent sulphuric acid and alcohol, they are described as acid-alcohol-fast bacilli or acid-fast bacilli (AFB). The acid-fastness is due to the fact that Myco. tuberculosis contains mycotic acid and lipids.
This organism consists of three fractions:
(1) Phosphatide which is soluble in ether;
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(2) Fat which is soluble in either;
(3) Wax which is soluble in chloroform and ether. There are four main serological groups: mammalian (human, bovine, murine), avian, reptilian and saprophytes.
Cultivation of Mycobacterium Tuberculosis:
Myco tuberculosis human type is aerobic, grows well at 37°C as late as 6-8 weeks (average 4 weeks) on selective media containing glycerol, whole egg or egg yolk (Dorest egg or Lowen-stein Jensen medium — in screw capped bottle, because screw cap can prevent the dehydration of these media during the long incubation period).
The growth is luxuriant (eugonic) and wrinkled at first while, later, buff coloured. As compared to Myco tuberculosis human type, the growth of bovine type is less luxuriant (dysgonic) thin, whitish and smooth. Glycerol favours the growth of human type, but has no effect on the bovine type.
Toxin Production:
Myco. tuberculosis does not produce an exotoxin. It contains toxic substances which are liberated when it is lysed. In 1890, Robert Koch isolated a substance known as “tuberculin” from tubercle bacilli. The tuberculin is an extract containing the specific protein of tubercle bacilli. The tuberculin reaction is due to the development of tissue hypersensitivity — or bacterial allergy — and is used in men and animals to find if they have or have had tuberculosis in an active or latent form.
Tuberculin is originally obtained from a six week old culture in glycerol broth evaporated to one-tenth of its volume, sterilised by heat and filtered (old tuberculin — OT). Various other methods have been employed in its preparation. The specific tuberculin protein can now be separated from other constituents and products of culture in a synthetic medium and then purified. This purified protein derivative (PPD) is preferable to old tuberculin as it is consistent in composition. It is used in dry state, from which it can be diluted by addition of a borate buffer solution.
It should be noted that the tuberculin prepared from the human and bovine types are indistinguishable as they contain the same specific substance. Tuberculin’s are standardised in such a way that a dilution of 1:10,000 of OT is equivalent to 1 tuberculin unit (TU), while 0.000028 mg of PPD equals 1 unit. A common practice in using tuberculin is to test first with 3 or 5TU and if the individual gives no reaction, retest with a dose of 100TU.
Tuberculin Test:
Tuberculin test in man is carried out on the skin by different techniques; those most commonly used are Mantoux, Heaf and jelly tests. Mantoux test is performed by injecting intradermally 0.1 ml of the appropriate dilution of tuberculin, the test is positive when there is an area of induration measuring 5 mm in diameter in 2-3 days after injection.
In Heaf test, a multiple puncture spring release gun is used to prick the previously applied tuberculin into the skin. A positive reaction may range from 4-6 discrete papules to solid induration. These two tests are usually done on the forearm. In the jelly test, a tuberculin jelly is applied in a form of “V” in the intra-scapular area of the back and covered with a plaster; the control is required only for this jelly test, but not with other two tests.
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Only Mantoux and Heaf tests were found to be reliable and acceptable to large scale epidemiological investigations, when compared to these tests and Von Pirquet scarification test; Heaf test is generally preferred to Mantoux test. The tuberculin test may be used in epidemiology to determine the incidence of tuberculosis infections in a community.
A positive reaction in a young child may also be useful for case finding among the family contacts; in immunization campaign in order to separate the positive and negative reactors and to assess the response to vaccination by simple testing afterwards.
The 38K Da protein of Myco:
Tuberculosis induces B and T cells response with high specificity for infection with Myco. tuberculosis and is a prime candidate for development of new diagnostic reagent for tuberculosis.
Resistance:
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Tubercle bacilli are most resistant to external effect as compared to other non-spore forming bacteria as a result of high lipid content (25-40 per cent). These organisms survive in the flowing water for over a year; in soil and manure up to 6 months; on the pages of books over a period of 3 months; in dried sputum for 2 months; in distilled water for several weeks and in gastric juice for 6 hours. They are rendered harmless at temperature ranging from 100 to 120°C.
The thermal death point is 60°C for 15-20 minutes. Pasteurization (62°C for 30 minutes or 80°C for 15-30 seconds) also kills these organisms. Many individual bacilli die when desiccated, a proportion of them may survive for several weeks or months, if protected from sunlight.
They are sensitive to exposure to sunlight and ultraviolet radiation; and ordinary daylight, even through the glass, has a lethal effect. They can resist 5 percent phenol for several weeks in putrefying materials and in sputum. Myco tuberculosis is sensitive to streptomycin, rifampicin, viomycin and cycloserine among the antibiotics and to Para amino salicylic acid (PAS), isonicotonic acid hydrazide (INAH or isoniazid thiosemicarbosone, methionamide and pyrazinamide among chemotherapeutic agents.
Pathogenesis and Disease in Man:
It has been shown that tuberculosis in human beings is caused by two types of Mycobacteria — the human type (Myco. tuberculosis) and the bovine type (Myco. bow’s).The human type is responsible for 90 per cent of the cases and the bovine type for remaining 10 per cent. The avian type may also be the cause of human tuberculosis in some cases on ingestion of insufficiently cooked chicken meat and eggs contaminated with Myco. avium.
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Infection with tuberculosis takes place through the respiratory tract by the droplets dust and sometimes per os through contaminated food stuffs and through the skin and mucous membrane. Intrauterine infection via placenta may also occur.
The most common form of primary infection with tubercle bacilli is a pulmonary lesion known as the primary complex or Ghon focus (not more than 5µ in diameter) in the terminal bronchioles or alveoli and the primary lesion may occur in any part of the lungs. From it, the organisms are carried by lymphatic drainage by the regional mediastinal lymph glands in which there may be progressive enlargement and involvement, followed by caseation and later calcification.
More generalized infection may follow the spread of organisms either by blood stream or the bronchi, resulting, respectively, in miliary or bronchopneumonia tuberculosis, usually with lesions in other organs besides the lungs, e.g., brain and meninges (tuberculosis meningitis), spleen, liver. Primary infections may also occur via the intestine with the involvement of the mesenteric glands or via the tonsils with secondary cervical adenitis, usually from ingestion of infected milk.
These forms of tuberculosis have become much rare now, as most milk supplies are pasteurized. Primary infection of the skin (Lupus vulgaris) is now a rare condition. Infection of skin lesions sometimes occurs from handling infected materials (laboratory workers and veterinarians) while intra-cutaneous Bacille Calmette-Guerin (BCG) vaccination may produce a form of primary complex with local skin lesion and an associated adenitis.
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While tuberculosis meningitis occurs characteristically as a complication of the primary lung lesion in very young children, it may also develop in older children and young adults as an apparently primary infection. Renal, bone and joint tuberculosis may be due to “seedling” of blood borne tubercle bacilli from the primary lesion in the lung or elsewhere.
The post primary (or adult) form of pulmonary infection is the most common form of clinical tuberculosis in which one or more lung lesions progress to caseation and cavitation and after involving the bronchial tree create a case of open tuberculosis. Tuberculosis ulcerations in the larynx and intestine, if they occur, are usually sequelae of pulmonary tuberculosis spread of infected sputum; similarly secondary infection of ureter, bladder, epididymis may follow renal tuberculosis.
Transmission of Tubercle Bacilli:
This organism is expectorated in sputum and expelled in droplets during coughing and speaking and there have been instances of explosive outbreaks of tuberculosis in school children and others exposed to an infective teacher or singer. But, since very small droplets that can be inhaled directly from the infective patient are less likely to carry tubercle bacilli than larger droplets or sputum, infection may occur more often indirectly from the dried dust particles than directly from moist droplets or droplet nuclei.
The tubercle bacilli can survive slow drying for days or weeks, if protected from the bactericidal day light or sunlight. The spread of infection from infected cases to susceptible contacts by contaminated dust or fomite would be facilitated in overcrowded, badly lit rooms or buildings. Primary infection may occur at any stage, if it occurs in early life (0-3 years) it is often associated with signs and symptoms of the disease. At school age (5-15 years), infection usually occurs in an in apparent form, but in adolescents or young adults, it is again more likely to result in clinical disease.
Infection occurs earlier and is more likely to result in clinical disease among susceptibles living in close contact with open cases, but many personal and environmental factors (age, malnutrition, other respiratory diseases, hormonal dysfunction, pregnancy, stress, genetic constitution etc.) may contribute to overt tuberculosis.
Workers exposed to the inhalation of dust containing silica have a high incidence of tuberculosis. Nurses, medical students, doctors and workers in the pathology laboratory and in the hospital are more exposed and tend to have a higher than an average rate of infection.
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In lung tuberculosis, the sputum is the main source of infection; in tuberculosis of kidneys and bladder, the bacilli may be excreted in the urine in plenty; in intestinal tuberculosis, faeces may contain tubercle bacilli and similarly, the pus in the tuberculosis abscess may also contain tubercle bacilli.
It has been estimated that a patient may discharge in 24 hours 500 millions to 3 billions bacilli in his sputum. A spray of sputum from a coughing or sneezing tuberculosis patient may contain tubercle bacilli may get infected with tuberculosis by coming in contact with the patient.
A careless person with active pulmonary tuberculosis may kiss a child, contaminate the floor or furniture with sputum or expectorate into the street or other public place which is a very common practice in under-developed countries. So, children playing in the street or room, creeping on the floor may get the tubercle bacilli on their hands and ultimately into their mouth. The hand to mouth transmission is very common among the children. In ancient times, children were also infected by drinking unpasteurized milk from tuberculosis cows.
While handling food, the fingers soiled with the sputum may contaminate the food. Flies which crawled on the tuberculosis sputum may contaminate the food. Improperly washed spoon, plate or common drinking cup may also carry the tubercle bacilli.
Tubercle bacilli are ubiquitous in nature. Particularly in urban area they are widespread in city streets, theatres and other public places and may result into an imbalanced urban health. Everything that keeps one person in good conditions (good food, sufficient rest, and recreation along with the chemotherapy) may pave ways towards the recovery and good health of the tuberculosis patients.
Measles, whooping cough, influenza, frequent child bearing, continuous strain and fatigue, poor living and unhygienic conditions, alcoholism and malnutrition are the predisposing causes of tuberculosis. Only the public health nurse can educate and influence the public to lead an healthy happy life.
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Socio-economic conditions are also related to tuberculosis in a community. Tuberculosis is very common amongst the poor people than among the rich people. Low standards of living are: lack of isolation, rest, sunlight, fresh air and cleanliness; lack of medical, nursing care and lack of sufficient nutritious food.
Tuberculosis and Nurses:
In the developed countries, there is an accumulated evidence that the nurses are more frequently infected with tuberculosis than women of same age in other occupation, which may be as a result of breakdown of resistance of the individual who had already been heavily infected in childhood. Tuberculin test carried out on a group of student nurses indicated that the primary infection is often acquired during the period of training, most probably from patients.
It has been shown that the incidence of tuberculosis among nurses is higher in general hospitals than in tuberculosis hospital and it seems that the nurses acquire the infection most frequently from unknown cases than from known cases as they take all possible precautions including the use of masks during nursing care of the tuberculosis patients.
Before admission into the school of nursing, each candidate should undergo the physical examination which is followed by the intradermal tuberculin test; if the test is positive, an X-ray chest is essential. This study may help the candidate to divert into less arduous occupation. Besides, the tuberculin test should be conducted at six months’ interval on nurses who were previously negative for tuberculin test and at the same time intervals chest plates should be done on all nurses who were tuberculosis negative.
Since overwork and excessive stress may break down the body resistance, the work load should be within the permissible limits and the general health condition should be supervised. For the safer side, all tuberculin negative student and nurses should be immunized with BCG before the commencement of the training.
Laboratory Diagnosis:
Direct microscopy of smears from sputum, pus, spinal or pleural fluid, urine, faeces, lymph nodes etc. stained by Ziehl-Neelsen method can reveal the presence of tubercle bacilli. Tubercle bacilli are plenty in lesions of rapid caseation. In miliary tuberculosis, they are relatively rare. Smears stained with aura mine can be examined by Fluorescence microscopy. Coughing may be induced by passing a swab onto the posterior pharynx, the expectoration on it can be used to prepare a smear.
Urine, pleural, peritoneal fluid are centrifuged, films are made from the deposit and stained by Ziehl- Neelsen method. In examining the urine, it is advisable to obtain the sediment from a 24 hours specimen because of intermittency of excretions of tubercle bacilli. Alternatively, three consecutive morning specimens may be examined. “Spider web” coagulum obtained from an hour longer standing CSF in a stoppered tube can be easily transferred to a slide, dried, fixed by heat and stained by Ziehl-Neelsen method.
Recent Polymerase Chain Reaction (PCR) is 95% sensitive and 93% specific when compared to culture, direct microscopy and gas chromatography. Very recently, molecular biological technology—DNA probe and polymerase chain reaction (PCR) — can detect even a single Myco. tuberculosis in clinical specimen with 100% sensitivity and specificity.
Complement fixation test (CFT), agar gel precipitation test, modified haemagglutination (HA) test had fallen behind Enzyme Linked Immunosorbent Assay (ELISA) in term of specificity and sensitivity. ELISA has become the method of choice. Radio-immunoassay (RIA) also gives promising results. Isolation of the pure culture can be done on Dorset egg or Lowenstein Jensen medium.
Chemotherapy:
The most effective and least toxic chemotherapeutic agents against tuberculosis are streptomycin, para amino salicylic acid (PAS) and isonicotinic acid hydrazide (Isoniazid). It is essential to have a combination of two or three anti-tuberculosis drugs, because of the appearance of resistant strains of tubercle bacilli after the commencement of treatment.
Vaccination:
Killed vaccines of tubercle bacilli give no protection against tuberculosis. In 1921, two French scientists, Calmette and Guerin, introduced a living vaccine prepared from a bovine strain that had been cultured for many years on a bile potato medium. This “Bacille Calmette-Guerin”(BCG) vaccine is given intradermally. It has been demonstrated conclusively that BCG vaccination may give a protection of 80 per cent against clinical infections.
The dose of BCG vaccine is a single intra-cutaneous injection of 0.1 ml containing 0.05 to 0.1 mg moist weight of the constituent bacilli (= 1-2 million viable organisms). A small inflammatory lesion develops at the site of inoculation and usually goes on to superficial ulceration after some weeks. Adenitis of the regional lymph nodes may follow in infants (rarely in older children). Except in case of babies of 0-3 months old, all children must first be tuberculin tested and only the negative reactors should be given vaccine.
Anonymous Mycobacteria:
Acid-fast bacilli which cannot be identified as either human or bovine type tubercle bacilli and which may be associated with human diseases have attracted considerable attention in recent years and have been given the name anonymous or atypical mycobacteria.
These mycobacteria were originally divided by Runyon into four groups according to their morphology, colonial texture, production of pigment in the presence of light (photo-chromogens or in the darkness) scoto-chromogens, rapidity of growth and growth at different temperatures (25°C = psychrophiles; 37°C = mesophiles; 42°C = thermophiles), production of catalase, resistance to certain anti-tuberculosis drugs, e.g., thiosemicarbosone, isoniazid and paraaminosalicylic acid (PAS) and the niacin test.
Later these four groups were further expanded to seven provisional groups of which Group I (Photochromogens) are in most common association with respiratory diseases, Group II (Scotochromogens) are found in neck abscess, Group IV is similar to Myco. avium and Group VII is associated with cases of pneumoconiosis.
Application to Nursing:
Well trained professional nurse should have a profound knowledge of the localisation of the lesion in the lungs, intestine, lymph nodes which is discharging living organism, so that she may be able to determine the types of preventive measures to be adopted according to the different situations. All patients with copious secretions should be regarded as possible causes of pulmonary tuberculosis.
The nurses in tuberculosis hospital who are responsible for the prevention of the transmission of infection from the infected materials (e.g., linens, thermometers, dishes, dust) to other people and to themselves should bear in mind that Myco, tuberculosis is very resistant to chemical disinfectants and drying.
The patient with active tuberculosis should be kept well informed by the nurse of the fact that he is a dangerous source of infection to others. Hence, he should remain isolated from other persons and should also he instructed not to cough or expectorate openly; if he moves out of the house, he should carry a pocket sputum flask, have separate set of dishes which should be washed in boiling water after use, should sleep isolated; if his hands are soiled with sputum, he should not forget to wash his hands; while coughing or sneezing, he should cover his nose and mouth.
Besides, the nurse must instruct that he should not come in contact with children under 5 years of age and other persons. In the public health nursing, the nurse gives training, care and encouragement to the patient in his own home. The sputum coughed up from the lungs of the patient in active stage of the infection must be completely disinfected or destroyed.
If the tuberculosis patient has copious pulmonary secretions, he should be directed to cough into a bag tied to the bedside stand or pinned to the bed. If this bag is full to certain extent, it should be folded at the top so that the paper wipes will not spill out, wrapped in the thick newspapers, placed into the trash cans; at last it should be taken to the incinerator when it should be completely burnt. The sputum cup, if used, can be disinfected satisfactorily, and special care should be taken to destroy all materials (sputum and secretions) in a close incinerator system to avoid the spillage.
The professional nurse in the tuberculosis hospital ward should be able to differentiate that Myco. tuberculosis causes tuberculosis and atypical mycobacteria cause other respiratory diseases which are less serious and should know how to interpret the report from the diagnostic microbiological laboratory and inform the doctor immediately if it is Myco, tuberculosis causing a serious communicable disease —tuberculosis.
