MCQs PATHOLOGY:Rickettsiae, Chlamydiae and Mycoplasmas

7.1 Which of the following is an obligate intracellular parasite? A. Bruce lia abortus B. Listeria monocytogenes C. Mycobacterium tuberculosis D. Neisseria meningitidis E. Rickettsia prowazeki

7.2 Which one of the following is NOT a property of the rickettsiae? F. All species have bacterial-type cell walls and contain both RNA and DNA G. Arthropods serve as natural host and transmission vector for most species H. Their growth is enhanced by sulphonamides and inhibited by PABA J. They are devoid of all enzymes active in intermediary metabolism K. They have a predilection for the endothelial cells of small blood vessels.

Answers
7.1 E. Rickettsia form one of the two groups of obligate intra-cellular bacteria, the other being the chlamydiae. Viruses form the other group of intracellular parasites but differ fundamentally in structure and biology from the minute bacteria which reproduce by binary fission, possess both DNA and RNA, and have a classical bacterial ultra-structure, some even possessing flagella. Like viruses, they fail to grow except in living tissue cells and with the exception of Coxiella burneti, survive poorly outside the host cell. Rickettsiae were named in honour of H. T. Ricketts who first described them. R. prowazeki is the causal agent of classical, epidemic, louse-borne typhus fever. Although the other organisms given as distractors are either frequently seen within cells or are truly intracellular parasites during infection, they can all be cultivated in a cell free medium.

7.2 J. Although rickettsiae generally lack the full complement of metabolic enzymes necessary for independent existence (R. quintana of trench fever can be cultivated on blood agar in 10 per cent C02), they do have an active metabolism including the enzymes of the Krebs cycle. The haemorrhagic manifestations characteristic of most infections are largely the result of widespread thrombosis caused by rickettsial growth in the endothelial cells of small blood vessels. R. prowazeki (epidemic typhus) is unusual in having man as the natural reservoir and the louse active only in ensuring its transmission. Coxiella burneti (Q-fever) is even more exceptional in lacking an insect vector in relation to man who is infected by direct contact or by inhalation of contaminated dust. The inhibitory effect of PABA is unrelated to folic acid metabolism but results from substrate competition with P-hydroxy benzoic acid which interferes with the synthesis of co-factors for electron transport.

QUESTION
7.3 Which of the following retains its infectivity on exposure to desiccation in contaminated dust? L. Coxiella burneti M. Mycoplasma pneumoniae N. Neisseria meningitidis O. Treponema pallidum P. TRIC agents.

7.4 Which property differentiates Coxiella burneti, the causal agent of Q-fever, from viruses? Q. Airborne transmission to man R. Arthropod vector transmission to animals S. Obligate intracellular parasitism T. Passage through 0.4 ìéç collodion filter U. Possession of both DNA and RNA.

7.5 Which property can differentiate chlamydiae from viruses? V. Ability to elicit delayed hypersensitivity W. Filterable nature of infectious unit X. Existence as obligate intracellular parasite Y. Formation of inclusion bodies in infected cells Z. Sensitivity to tetracyclines.

Answers
7.3 L. Coxiella burneti differs from rickettsiae by its resistance to drying and relative heat resistance. Thus it can be transmitted in farmyard dust from infected cattle and sheep and can be transmitted in contaminated milk submitted to the standard pasteurization procedure. Human infection is not arthropod-borne.

7.4 U. Viruses contain either DNA or RNA but never both. Other characters absent from viruses include bacillary morphology with murein and endotoxin in cell wall, binary fission, sensitivity to tetracycline and chloramphenicol and possession of enzyme systems of Krebs cycle, electron transport and protein synthesis.

7.5 Z. Chlamydiae are inhibited not only by the tetracyclines but also by chloramphenicol, streptomycin and with some strains, sulphonamides: viruses are uniformly resistant. Other differential features they possess include a bacterial-type cell wall with mucopeptide, both DNA and RNA, ribosomes, metabolic enzymes and multiplication by binary fission. The other properties listed are common to both chlamydiae and viruses.

QUESTION
7.6 Which one of the following is NOT a property of the mycoplasmas? A. Convert to bacteria when subcultured on antibiotic-free medium B. Colonies adhere and burrow into solid culture media C. Identification is achieved by serological growth inhibition D. Procaryotic cells lacking a rigid cell wall. E. Susceptible to 56°C for 30 minutes.

7.7 Which one of the following is NOT a cell wall deficient form of bacteria? F. L-form G. Mycoplasma H. Protoplast J. Rough mutant K. Spheroplast.

Answers

7.6 A. Although mycoplasmas are procaryotic cells lacking a rigid cell wall, they are not cell wall deficient bacteria and therefore can never revert to a bacterial form with a muco-peptide rigid wall as can an unstable L-form. They possess a classical cytoplasmic unit membrane and are very heat labile. When cultured on osmotically protected media, such as 20 per cent serum agar, growth is seen within 2—5 days. Identification is established by placing a range of different specific antiserum impregnated discs on a lawn of the test organism and after incubation the particular species is recognized by the inhibition of growth produced by the homologous antibody. Smears stained by Giemsa method clearly show the pleomorphic form, with filaments, cocci, globules and ring forms. The finer coccal forms may pass the normal 0.45 ì bacterial filter.

7.7 G. With the exception of mycoplasmas all the other organisms are undoubtedly of bacterial nature and represent varying degrees of loss of cell wall. A rough mutant has lost its outer layer of smooth antigenic determinants exposing the rough determinants of the basal core polysaccharides. A protoplast and an L-form have lost their entire cell wall and are left with only the cytoplasmic membrane which makes them somewhat fragile and osmotically sensitive. Spheroplasts are virtually restricted to the Gram-negatives where they represent cells that have lost their rigid muco-peptide layer but retain the outer coat of lipopolysaccha-ride somatic antigen. Mycoplasmas are very similar to permanent L-forms which lack the capacity to revert to the bacillary form, but differ in DNA base ratio, possess sterols in their cell membrane and are in certain cases, pathogenic to man and animals, which is still not proven for L-forms