MCQs PATHOLOGY: Mycobacteria, Nocardia and Actinomyces

3.1 Which organism is "acid fast"? A. Bacteroides fragilis B. Clostridium welchii C. El Tor Vibrio D. Lactobacillus acidophilus E. Mycobacterium smegmatis


3.2 Which group of organisms typically resist decolorization with an acidic solution of alcohol after being stained vigorously with a heated concentrated solution of a basic dye? F. Chlamydiae G. Mycobacteria H. Mycoplasmas J. Rickettsiae K. Spirochaetes


3.3 Which mycobacterium may fail to grow from clinical specimens seeded onto Lowenstein-Jensen medium yet will grow readily on Dorset egg medium? L. M. N. O. P. M. avium M. bo vis M. bouis var. BCG M. microti M. tuberculosis (avian bacillus) (bovine bacillus) (attenuated bovine bacillus) (vole bacillus) (human bacillus)


3.4 Which laboratory examination will distinguish with certainty Mycobacterium tuberculosis from Mycobacterium bovis? Q. Antibiotic sensitivity pattern R. Histology of the clinical lesion S. Pathogenicity for guinea-pig T. Pathogenicity for rabbit U. Ziehl-Neelsen film.


3.5 Niacin (nicotinic acid) production is a characteristic distinguishing property of which mycobacterium? V. Mycobacterium avium W. Mycobacterium balnei X. Mycobacterium bovis Y. Mycobacterium kansasi Z. Mycobacterium tuberculosis.


3.6 What statement about atypical mycobacteria is FALSE? A. Culture characteristics differ from those of M. tuberculosis B. Human to human transmission rarely if ever occurs C. Runyon classified them into Groups I—IV D. They are opportunistic pathogens for man E. They are more antibiotic sensitive than M. tuberculosis


3.7 Which statement about Mycobacterium tuberculosis is FALSE? F. Desiccation in sputum results in loss of virulence G. Exposure to UV light is lethal H. Killed by heating to 62°C for 30 minutes J. Relatively resistant to disinfectants K. Unusually resistant to acids and alkalis.


3.8 Which pathogen does NOT satisfy "Koch's postulates"? L. Bacillus anthracis M. Clostridium te tan i N. Corynebacterium diphtheriae O. Mycobacterium leprae P. Salmonella typhi.


3.9 Which one of the following properties is possessed by nocardia but NOT by actinomyces and is thus a simple basis for their differentiation? Q. Ability to grow in air R. Branching morphology S. Formation of hyphae-like filaments T. Gram-positive stain reaction U. Procaryotic nucleus.


3.10 Which statement about Actinomyces israeli is FALSE? V. Appears as branching mycelium or diphtheroid forms W. Classified as a bacterium, not a fungus X. Colonies in tissue show peripheral radial "clubs" Y. Man is the main reservoir of infection Z. The organism is a strict aerobe.


3.11 Which statement about Nocardia astéroïdes is FALSE? A. Acts as an opportunistic pathogen in man B. Classified as a fungus C. Forms rods and branching filaments D. Produces aerial hyphae in culture E. Stains Gram-positive and may be weakly acid-fast.


3.12 Which one of the following organisms, transmitted by rats is notable for its ability to grow in the baciUary phase or as naturally occurring cell-wall deficient L forms? F. Leptospira icterohaemorrhagiae G. Rickettsia mooseri H. Spirillum minus J. Streptobacillus moniliformis K. Yersinia (Pasteurella) pestis


ANSWERS


3.1 E. Any mycobacterium is acid-fast although sapro-phytic species such as M. smegmatis are not fully acid and alcohol fast. Acid fastness is thought to be associated with their thick hydrophobie cell wall, rich in mycolic acid and complex waxes. The cells resist simple staining procedures, but stain if treated vigorously with heated concentrated solutions. Once stained they resist decolorization with 20 per cent sulphuric acid whereas non-acid fast organisms lose the stain and take up a contrasting counter stain. The staining method employed is that of Ziehl-Neelsen.


3.2 G. Mycobacteria possess a thick cell wall rich in mycolic acid and other complex lipids. This makes the cell wall hydrophobic and impermeable to aqueous stains. When treated by Gram's method mycobacteria remain unstained. Ziehl-Neelsen introduced a satisfactory staining method based upon staining with a concentrated solution of carbol fuchsin for 5 minutes, aided by a short application of heat. When stained these organisms resist decolorization with strong (20 per cent) sulphuric or nitric acid, and for the pathogenic species, the addition of alcohol. The action of acid removes all red stain from the tissues and non-acid fast bacteria. Treatment with the counter stain produces a contrasting blue or green colour against which the red acid-fast bacilli are clearly seen. This stain is widely used in diagnostic microbiology to identify mycobacteria.


3.3 M. On primary isolation M. bovis may be partially or completely inhibited by glycerol in an L—J slope. This allows the human and bovine tubercle bacilli to be distinguished on culture, the human strain M. tuberculosis being stimulated by glycerol to give a luxuriant, eugonic growth whereas the bovine strain M. bovis may grow poorly as a sparse, dysgonic growth or perhaps not at all. Dorset egg medium lacks glycerol and is not inhibitory for M. bouis or other mycobacteria. The BCG strain is well adapted to artificial culture


3.4 T. The essential biological difference between the human and bovine bacillus is their virulence for the rabbit. The bovine bacillus is highly virulent for rabbits by any route of inoculation and the human only slightly so. If the rabbit survives for six weeks and an autopsy shows lesions only in the lungs or kidneys the strain is Mycobacterium tuberculosis. With Mycobacterium bovis the rabbit may die before this time or if not it should be killed. At autopsy a progressive generalized tuberculosis will be found.


3.5 Z. Evidence of niacin production is a useful laboratory criterion for identification of Mycobacterium tuberculosis. M. ulcerans is the only other mycobacterium that produces niacin and this property helps to distinguish it from the "swimming pool disease" pathogen M. balnei (M. marinum) which, like M. ulcerans grows at 33°C. M. ulcerans only rarely infects man.


3.6 E. The atypical mycobacteria are noted for their high degree of drug resistance compared with most strains of M. tuberculosis. Human infections by them often require combined surgery and chemotherapy for cure. On culture group I strains (e.g. M. kansasi) develop a bright yellow pigment only if exposed to light. Group II strains (M. scrofulaceum) produce an orange or yellow pigment independent of light. Group III strains (e.g. M. intracellulare — the Battey bacillus, related to the avian tubercle bacillus) produce little or no pigment. Thus they are called photo-chromogens, scotochromogens and nonchromogens respectively. Group IV strains (e.g. M. fortuitum) alone are rapid growers producing well developed colonies in less than 1 week whereas Groups I—HI strains require 1—3 weeks or longer for growth. Atypical mycobacteria have caused pulmonary, lymph node, bones and joints and disseminated infections in man, mostly due to M. kansasi and M. intracellulare, but the incidence in Britain is small.


3.7 F. Tubercle bacilli remain viable and virulent for weeks or months in dried sputum unless exposed to direct sunlight whose UV component has a strong bactericidal effect. Mycobacteria are highly sensitive to moist heat and the process of low temperature pasteurization at 62°C for 30 minutes is effective, although now generally replaced by " flash'' pasteurization at a high temperature for a fraction of a minute. Pasteurization of milk has virtually eradicated bovine tuberculosis from Britain.


3.8 O. Mycobacterium leprae is an obligate human parasite which cannot be cultivated in artificial media. There is however no doubt that it is the causative organism of leprosy and it has been grown in experimentally immuno-deficient mice and hamsters. The aetiological role of the other organisms to their respective diseases has been firmly established, fulfilling all of Koch's postulates, from extensive animal pathogenicity studies and also as a result of accidental laboratory infections in man.


3.9 Q. Although superficially similar in many respects nocardia are clearly aerobic and actinomyces anaerobic or microaerophilic. Both are opportunistic pathogens of man, producing abscesses and multiple sinuses with release of colonies described as "sulphur" granules in the pus. Actino-mycosis mainly affects the neck and jaw, lungs, and the abdomen. Nocardiosis may affect the chest and brain.


3.10 Z. Actinomyces israeli is either anaerobic or microaerophilic and will not grow aerobically. Although it forms hyphae-like filaments it is a procaryotic cell with typical bacterial mucopeptide. The radial eosinophilic hyaline "clubs" formed in tissues result from host-parasite interactions and are not specific for actinomycotic infections


3.11 B. Nocardia are typical procaryotic bacterial cells taxon-omically closely related to the mycobacteria and less closely to the corynebacteria. Morphologically, the branching filaments resemble the actinomyces group although nocardia are strict aerobes whereas the actinomyces are anaerobic or microaerophilic. The colonies are often very similar to those of the mycobacteria, but differ by producing aerial hyphae which gives the colony a powdery appearance. Although the cells may stain "acid fast" they are less resistant to decolorization than the mycobacteria. Nocardia astéroïdes produces opportunistic infections in man only when the host defences are severely compromised by disease or immunosuppressive therapy. The commonest form of infection is a brain abscess secondary to a pulmonary focus.


3.12 J. Streptobacillus moniliformis is present in the mouth of the wild rat and is transmitted to man by a rat bite (one form of rat bite fever) or by consumption of rat contaminated milk (Haverhill fever). The name is derived from its tendency in culture to grow as a mixture of chaining Gram-negative bacilli interspersed with a yeast-like mass of swollen filaments and globular forms, often filled with minute granules. This globular form, first described by Klieneberger as the symbiotic growth of a pleuro-pneumonia- like-organism (PPLO) was later recognized by Dienes as a cell wall deficient bacterial phase usually capable of reversion to the bacillary form. Thus the term L form was introduced after the Lister Institute where Klieneberger made her discovery