MCQs PATHOLOGY: Gram-positive Bacilli

2.1 Which one of the following is NOT a property of Corynebacterium diphtheriae? A. "Chinese letters" arrangement of cells B. Gram-positive bacilli exhibiting snapping-division C. No fermentation of glucose, maltose or sucrose D. Resistance to tellurite E. Volutin granules


2.2 Which statement about Corynebacterium diphtheriae is FALSE? F. Avirulent strains cannot stimulate protective immunity G. Cells rich in interferon are resistant to toxin H. Relationship between colonial type and virulence is variable J. Toxoid immunization is indicated for Schick negative reactors K. Virulent strains are invariably toxigenic.


2.3 For which organism is the demonstration of tumbling motility in cultures incubated at room temperature (about 22°C) a useful criterion for distinguishing it from organisms of similar morphology? L. Corynebacterium diphtheriae M. Corynebacterium ulcerans N. Erysipelothrix rhusiopathiae O. Listeria monocytogenes P. Propionibacterium acnes. Answers overleaf


2.4 Which is NOT a property of Bacillus an thracis? Q. Aerobic R. Capsulated S. Gram-positive bacillus T. Motile U. Spore bearer.


2.5 Which organism has a distinctive colonial morphology described as "Medusa-head"? V. Aspergillus fumigatus W. Bacillus an th racis X. Klebsiella pneumoniae Y. Pseudomonas aeruginosa Z. Rough strains of Streptococcus pneumoniae.


2.6 Which clostridium very rarely shows spores either on culture or from clinical material and yet is highly heat resistant, surviving several hours boiling? A. Clostridium histolyticum B. Clostridium oedematiens C. Clostridium sporogenes D. Clostridium tetani E. Clostridium welchii


2.7 What is NOT a biological property of Clostridium welchii? F. Alpha ôï÷ßç neutralization is the basis of the Nagler reaction G. Causal relationship to gas gangrene and food poisoning H. Food poisoning strains are usually non-haemolytic J. Spores of classical type A strains are killed by 5 min boiling K. Types A — E are identified by capsular antigenic specificity.


2.8 Which statement about Clostridium welchii is FALSE? L. Always capsulate in tissues M. Beta-haemolysis on horse blood agar due to theta toxin N. Motile under anaerobic conditions O. Saccharolytic, with abundant gas production P. "Stormy-clot" fermentation of milk medium.


2.9 Which type of Clostridium welchii is specifically associated with enteritis necroticans in man? Q. Type A R. Type B S. Type C T. Type D U. Type E.


2.10 Which test is the standard method used for identification of Clostridium welchii? V. Alpha-toxin neutralization on serum or egg yolk plate W. Biochemical reactions including "stormy-clot" reaction X. Pathogenicity test with paired protected and unprotected animals Y. Pattern of antibiogram Z. Slide agglutination with type specific antiserum.


2.11 Which one of the following clostridia is flagellate, has round terminal spores and possesses proteolytic but no saccharolytic properties? A. Clostridium histolyticum B. Clostridium septicum C. Clostridium sporogenes D. Clostridium tetani E. Clostridium welchii. Answers


2.12 Which organism is a motile, Gram-positive bacillus that swarms on horse blood agar when incubated under optimum conditions? F. Bacillus anthracis G. Clostridium tetani H. Clostridium welchii J. Klebsiella pneumoniae K. Proteus mirabilis, Answers overleaf 85


ANSWERS

2.1 C. This short fermentation set is used to separate pathogenic Corynebacterium diphtheriae from non-pathogenic C. xcrosis and C. hofmanni. C. diphtheriae ferments glucose and maltose but not sucrose. The typical cell morphology is seen in methylene-blue or Albert's stained smears from cultures on Loeffler medium. Typically they are seen as delicate slightly curved beaded or barred rods with 2 or 3 metachromatic volutin granules per cell. Snapping, or incomplete division, is responsible for the characteristic "Chinese letters" arrangement of cells. The characteristic colonial differences of the three epidemiological types is seen after culture on blood-tellurite medium, used for their selective isolation. Gravis strains form large grey-black daisy-head colonies, intermedius strains form minute flat black colonies and mitis strains form medium sized grey-black glistening colonies with a central elevation which gives them the appearance of a "poached egg".


2.2 J. A positive Schick test identifies individuals with inadequate existing immunity to diphtheria, as shown by the appearance of induration and redness at the site of injection of 0.5 ml purified standard diphtheria toxin 3—5 days after inoculation. A positive reaction demonstrates the individual has less than 0.03 units/ml of circulating antitoxin and is therefore likely to be susceptible to the disease. Heat inactivated toxin injected concomitantly into the opposite arm detects allergic false positive reactions.


2.3 O. Except for Listeria monocytogenes, the organisms listed are nonflagellate and nonmotile so that a positive motility test can at once exclude them, although a negative reaction does not exclude L. monocytogenes.


2.4 T. Bacillus anthracis is non-flagellate and non-motile whereas all the other anthracoids (e.g. B. cereus, B. subtilis and B. megaterium) are flagellate and highly motile which is a useful, simple, differential character.


2.5 W. The anthrax bacillus grows as a compact colony with wavy outline and rough-looking surface. Examination with a low power lens shows it is composed of tangled masses of filaments and twisted cord-like projections of undivided and unseparated bacilli. This mass of interlaced filaments gives the impression of a tangled mass of curly hair, hence the terms, "Medusa-head" and "judge's wig".


2.6 E. Clostridium welchii typically fails to show obvious spores except in rare instances or when grown on special spore-forming media. Heat resistant strains responsible for most cases of Cl. welchii food poisoning may survive up to six hours boiling and yet show no obvious spores.


2.7 K. The five types of Clostridium welchii (A—E) are distinguished by the major lethal, minor lethal and non-lethal toxins they produce. All strains produce the major lethal toxin alpha toxin (lecithinase C) which is useful for identification of Cl welchii by the Nagler reaction but does not help to separate the different types.


2.8 N. Clostridium welchii is rare among the clostridia in being nonflagellate and nonmotile. Clostridial motility is in fact tested under anaerobic conditions. The capsule is antiphago-cytic and acts as an aggressin, aiding invasiveness. Theta toxin is prominent in type A strains but is usually absent from heat-resistant food-poisoning isolates which lack this haemolytic activity, a feature of value in their recognition.


2.9 S. Enteritis necroticans may follow consumption of food heavily contaminated with type C strains and must not be confused with the relatively mild, self-limiting toxic food poisoning caused by classical type A strains, or its specially heat-resistant variant. Type C strains produce an extensive though patchy necrosis of the small bowel, with bloody diarrhoea and possibly a fatal outcome.


2.10 V. This is the basis of the Nagler reaction for identification of Clostridium welchii. Opalescence produced by the action of lecithinase activity of alpha toxin is specifically neutralized by antitoxin. The only exception is given by CL bifer-mentans which has a serologically related lecithinase also specifically inhibited by CL welchii alpha antitoxin. The "stormy clot" reaction in milk is characteristic but not specific of CL welchii.


2.11 D. With the exception of Clostridium tetani all possess both proteolytic and saccharolytic properties in differing degrees or central or subterminal spores whereas Clostridium tetani has highly characteristic terminal "drumstick" spores. Clostridium welchii is exceptional in lacking flagella and by only very rarely showing spores unless specially cultivated on a sporing medium


2.12 G. Clostridium tetani alone fits the description. When cultivated anaerobically, it swarms as a fine, almost invisible veil over the surface of the plate. P. mirabilis is even better known as a motile swarmer, but of course it is Gram-negative. The other organisms are all non-flagellate and cannot swarm. B. anthracis is an aerobic Gram-positive spore bearer; Cl welchii is an anaerobic Gram-positive spore bearer; K. pneumoniae is a Gram-negative bacillus