MCQs PATHOLOGY: Bacterial Physiology and Genetics

3.1 Which one of the following would NOT be found in a pure bacterial culture? A. Capsulated and non-capsulated cells B. Cells of more than one strain C. Cells with different flagellar antigens D. Smooth and rough colonies E. Spores and vegetative cells.


3.2 Which technique allows enumeration of living bacteria in a liquid culture? F. Assay of cell mass by nephelometry G. Estimation of DNA concentration H. Microscopic count J. Miles-Misra plate count K. Weighing of total cell mass.


3.3 Which one is characteristic of the exponential (log) growth phase? L. High resistance to cell-wall antibiotics M. Low oxygen consumption N. Synchronous division O. Unbalanced growth P. Viable count is close to total count.


3.4 Which is most closely associated with the stationary phase of growth? Q. Depletion of critical metabolite R. Exponential growth S. Greatest oxygen consumption T. Metabolic inactivity U. Shortest generation time.


3.5 Absence of which enzyme from most anaerobic bacteria accounts for the lethal effect of oxygen on strict anaerobes? V. Catalase W. Coagulase X. Oxidase Y. Hyaluronidase Z. Urease.


3.6 Which factor inhibits the germination of tetanus spores in the tissues? A. Calcium ions introduced with contaminating soil B. Devitalized and necrotic tissue C. High local oxidation-reduction potential D. Presence of foreign bodies E. Toxins of contaminating Clostridium welchii.


3.7 For what reason is penicillin lethal to bacteria yet essentially non-toxic for mammalian cells? F. High population density of host cells neutralize its effect G. Mammalian cells are impermeable to penicillin H. Mammalian cells inactivate penicillin J. Penicillin does not affect cells with long generation time K. Target site is unique to bacteria.


3.8 Which statement about the action of penicillin is FALSE? L. Gram-negative bacilli are usually resistant M. Interferes with normal murein synthesis N. Mammalian cells unaffected O. Most effective against resting organisms P. Serious hypersensitivity reactions may occur.


3.9 Which one of the following constituents is ABSENT from Lowenstein-Jensen medium used for the cultivation of mycobacteria? Q. Agar R. Eggs S. Glycerol T. Malachite green U. Potato starch


3.10 For what reason are fluid enrichment media used in clinical microbiology? V. To dilute inhibitory substances in the specimen W. To prevent multiplication during transit X. To provide essential nutrients for fastidious species Y. To selectively cultivate pathogens originally few in number Z. To stimulate the growth of all enteric bacteria.

3.11 Which organism is most likely to be resistant to penicillin G? A. Clostridium tetani B. Neisseria meningitidis C. Staphylococcus aureus D. Strep tococcus pyogenes E. Treponema pallidum


3.12 Which one of the following is the active agent in bacterial transformation? F. Chemical mutagen G. Episome H. Extracted or released DNA J. Temperate bacteriophage K. Virulent bacteriophage.


3.13 Which one of the following when added to a culture of non-encapsulated pneumococci derived from a type 1 strain will result in biosynthesis of type 3 polysaccharide? L. Bacteriophage specific for type 3 strain M. DNA extracted from virulent type 3 strain N. Killed rough mutants from type 3 strain O. Specific opsonin against type 3 strain P. Type 3 polysaccharide.


3.14 In bacterial transformation, how much of the donor genome is usually integrated with the DNA of the recipient cell? Q. About half R. All of it S. A small fraction T. Most of it U. None of it.


3.15 By what mechanism does j3-phage control toxin production in Corynebacterium diphtheriae? V. Conjugation W. Lysogenic conversion X. Random mutation Y. Transduction Z. Transformation.


3.16 Which one of the following is an example of lysogenic conversion? A. Elimination of temperate phage B. Induction of the lytic phase C. Production of immunity to superinfection D. Prophage controlled toxogenicity E. Replication of virulent phage.


3.17 Which description applies specifically to Hfr cells? F. Cells capable of conjugation G. Cells with F pili H. Cells with integrated F factor J. F+ cells K. Donor cells in genetic transfer


3.18 What statement does NOT apply to bacterial plasmids? L. Associated with genetic transfer by conjugation M. Exist as self-replicating extra-chromosomal DNA N. Found only in Gram-negative species O. May be transferred between different genera P. Sometimes confer multiple antibiotic resistance

ANSWERS


3.1 B. By definition, a pure culture must contain only one strain. The other answers are false, as in every case the variant forms can arise as a result of mutation, selection and pheno-typic variation. Sporulation is dependent upon the stage of growth and maturation of the individual cell and it is usual to find mixtures of vegetative and sporing organisms in a pure culture of clostridium or anthracoid. To obtain a pure culture it is necessary to pick out a typical colony and sow it on a new medium. By repeating this process once or twice it is usually possible to separate the colony forming unit from possible contaminating organisms of a different strain.


3.2 J. This technique measures only the number of colony-forming units and is called a "viable count". Error arises from multiplication without separation (e.g. streptococci) aggregation after separation (e.g. mycobacteria and coryne-bacteria) and from technical errors.


3.3 P. During the exponential phase the number of dead or dormant cells is negligible and the entire culture is actively, although not synchronously, dividing. This requires a high oxygen consumption for active metabolic function and growth is balanced with the growing cell walls at their most vulnerable to cell wall active antibiotics (e.g. penicillin).


3.4 Q. Bacterial populations cease to increase either when they exhaust one of the ingredients essential for growth or when some metabolite or a combination of metabolites, reaches an inhibitory concentration in the medium. It is most unlikely that simple overcrowding is of any significance in limiting growth in liquid culture. 10


3.5 V. Although anaerobic organisms cannot utilize oxygen as a terminal electron acceptor they do possess the early part of the respiratory chain of enzymes through which hydrogen is combined with oxygen to form H202. Most aerobic organisms contain the enzyme catalase which degrades H202 into H2 and 02, but anaerobes generally lack catalase which allows the accumulation of H202 to a lethal concentration.


3.6 C. High oxidation-reduction potential would prolong dormancy. All the other factors listed decrease the oxidation- reduction potential and promote germination.


3.7 K. Penicillin acts by interfering with the formation of the rigid cell wall mucopeptide which is unique to bacteria. Mammalian cells lack this target component which gives penicillin a high degree of selective toxicity for bacteria. Harmful effects of penicillin for man are due to its ability to act as a hap ten, thus inducing hypersensitivity.


3.8 O. Penicillin inhibits cross-linkage of glycopeptide during synthesis but is without effect on existing murein. The susceptible rigid layer of Gram-negative species is often protected by outer layers of lipopolysaccharide. Mammalian cells uniformly lack murein substrate. Penicillin and its degradation products may bind to proteins and act as allergens.


3.9 Q. L—J slopes are prepared by inspissating egg — mineral salts — potato starch — asparagine — Malachite green and glycerol. The coagulated egg protein provides a solid medium without any need for agar. 12


3.10 Y. Enrichment media are used when it is required to isolate a specific pathogen from large numbers of other bacteria present in the same specimen which might otherwise prevent detection of the pathogen. Such conditions are met in the standard examination of faeces, for intestinal pathogens of the salmonella or shigella genera. By inoculating with faeces a broth medium containing selenite (or brilliant green) commensal Escherichia coli is suppressed and any salmonellae or shigellae present multiply without inhibition thus producing an absolute increase in their numbers compared with the inhibited E. coli. On subculture to a solid medium the enteric pathogen will form a greater proportion of the inoculum and will therefore develop sufficient colonies to ensure its detection and successful isolation. When such a selective agent is added to a solid medium it again aids the isolation of the pathogen by partial inhibition of contaminating organisms but it does not increase the numbers of the pathogen as estimated by numbers of colony forming units. By combining these two methods it is possible first to obtain an absolute increase in the pathogen by culture in an enrichment broth and then to improve its isolation and selection by subculture to a solid selective medium from which isolation in pure culture can be made.


3.11 C. With the exception of Staphylococcus aureus all the other organisms are likely to be penicillin sensitive. In the early days of penicillin most strains of Staph. aureus were also sensitive but due to widespread use of the drug sensitive strains have been greatly reduced and replaced by resistant strains by a process called emergence of resistance. It is now known that the ability to produce penicillinase can be transferred by transduction, often involving transmission of a plasmid


3.12 H. Griffiths first described transformation during his studies on Strep, pneumoniae. He injected 3 groups of mice; the first with living non-capsulated cells; the second with dead capsulated cells; the third with living non-capsulated cells plus killed capsulated cells. The first two groups were unaffected but the third group died. He later recovered capsulated cells from the dead mice. Avery, McLeod and McCarty (1943) identified the transforming agent as DNA. Transformation results from transfer of DNA containing the specific gene, absent from the living non-capsulated organism, which controls capsule formation.


3.13 M. This is genetic transfer by naked DNA transformation. Fragments of donor DNA containing about 20 genes pass through the cell wall and become integrated with the DNA of competent recipient cells which then acquire the genetic trait coded for in the fragment, in this example, biosynthesis of polysaccharide type 3.


3.14 S. When the donor cell disintegrates, its circular genome breaks into approximately 100 pieces, each containing about 20 genes. Recipient cells which are competent accept one of these fragments which is only about 1 per cent of the donor genome. Different cells will be transformed in different ways depending upon which pieces of donor DNA they receive so that the frequency of transformation for any one gene is usually about 1 per cent of the recipient population.


3.15 W. C. diphtheriae can synthesize its protein exotoxin only if infected by a temperate bacteriophage (0-phage). Whereas virulent phage replicate and cause lysis, temperate phage can either do the same or integrate with the DNA of the host bacterium and replicate with the bacterium as a pro-phage, without lysis. · It is in this phase that the temperate phage exerts its control over some of the functions of the cell, a phenomenon termed lysogenic conversion. Any strain not lysogenized is non-toxigenic and therefore avirulent.


3.16 D. A bacterium which carries a non-infectious form of phage is said to be lysogenic, and the phage termed pro-phage or temperate phage. Rarely they do exert a controlling influence on specific biochemical activities of the host cell, and when this happens the process is termed lysogenic conversion. Important examples of lysogenic conversion are the control of diphtheria and scarlatina toxins and alterations in somatic antigen specificities.


3.17 H. Any bacterial cell which carries an F (fertility) factor is termed F+ and those which lack it, F". The F+ cells are capable of transferring their F factor to F" recipient cells by conjugation via the sex pilus produced by the action of the F factor. This facilitates the transfer of the F factor to F" cells which become F . The F factor exists as an extra-chromosomal piece of DNA or plasmid in most cells but in a few, only one in 105 cells, it is integrated with the host DNA. Such cells have the ability to transfer the entire chromosome which results in recombination. They are termed Hfr strains, for high frequency of recombination, and their progeny are also Hfr cells


3.18 N. This statement is false as plasmids have been described in many Gram-positives, although in these, transfer can only be achieved by transduction as conjugation is restricted to Gram-negative species. By definition they never integrate with the nucleus of the cell although a plasmid in one cell may become an integrated episome in another.