Everything you need to know about medical microbiology. Some of the frequently asked questions are as follows:-
Q.1. Differentiate between Staphylococcus species and Streptococcus species.
Ans. There are two main distinguishing features between Staphylococcus and Streptococcus species:
The Staphylococcus species produce catalase enzyme, whereas Streptococcus species are catalase negative. Staphylococcus species are usually found in grape like clusters, whereas Streptococcus species tend to form pairs and chains.
Q.2. How are the different clinically relevant species of Staphylococcus distinguished?
Ans. The three main clinically relevant Staphylococcal species include: S. aureus, S. epidermidis and S. saprophyticus. S. aureus can be distinguished from the other species of Staphylococcus mainly by its ability to produce coagulase enzyme. The other two species which are coagulase negative are distinguished by susceptibility to antibiotic novobiocin: S. epidermidis is novobiocin susceptible and S. saprophyticus is novobiocin resistant.
Q.3. What are toxins?
Ans. A toxin is a poisonous substance, often a metabolic product of the organisms that damages the host.
Q.4. What do you understand by dimorphic fungi?
Ans. Those fungi which are capable of growing either as yeast like cells or as mycelia, depending on the environmental conditions is known as dimorphic fungi. When dimorphism is temperature dependent, the fungi are designated as thermally dimorphic. Many of the pathogenic fungi such as Histoplasma capsulatum, Blastomyces dermatitis’s are dimorphic.
Q.5. What is satellitism?
Ans. It is a phenomenon that helps in the recognition of Haemophilus species. These bacteria require both hemin (X factor) and NAD (V factor) for growth, and thus are unable to grow on 5% sheep blood agar media, which contains hemin but no NAD.
Many bacterial species such as Staphylococcus aureus, are able to produce NAD. On sheep blood agar tiny colonies of Haemophilus species may be seen growing or engaging in “satellitism” around the V factor- producing organism; this phenomenon is known as satellitism.
Q.6. Mention the names of microorganism, for which the following culture media are selectively used.
Ans. (i) Mannitol salt agar
(ii) Thiosulfate bile salt sucrose agar
(iii) Modified Thayer martin medium
(iv) Polymyxin lysozyme-EDTA-thallus acetate medium
(v) Tinsdale agar
Names of microorganisms suited to above media are:
(vi) Staphylococcus aureus
(vii) Vibrio cholerae
(viii) Neisseria gonorrhoeae or Neisseria meningitidis
(ix) Bacillus anthracis
(x) Corynebacterium diphtheriae
Q.7. Write down the full form of following terms:
(i) ELISA
(ii) RIA
(iii) STORCH
(iv) FIA
(v) MIC
(vi) VDRL
Ans. (i) ELISA Enzyme Linked Immunosorbent Assay
(ii) RIA Radioimmunoassay
(iii) STORCH Syphilis, Toxoplasma, rubella, cytomegalovirus, and herpes
(iv) FIA Fluorescent Immunoassay
(v) MIC Minimal Inhibitory Concentration
(vi) VDRL Venereal Disease Research Laboratory Flocculation Test
Q.8. Mention the names of commonly used anaerobic media.
Ans. The primary plating media for inoculating anaerobic specimen are:
1. Anaerobic blood agar
2. Bacteroides bile esculin agar media
3. Laked kanamycin-vancomycin blood agar media
4. Anaerobic phenylethyl agar media
Q.9. What is Polymerase Chain Reaction (PCR)?
Ans. PCR is a patented technology that generates multiple copies of a specific nucleotide sequence from a target organism. PCR is used to detect extremely low concentration of target organisms with high specificity. PCR is closely patterned after the natural principle of DNA replication. In its rudimentary form PCR is a three step process referred to as cycle, that is repeated a specified number of times.
One PCR cycle consist of the following steps:
(a) Denaturation of target nucleic acid
(b) Annealing of primers
(c) Extension of primers
Q.10. What is normal flora?
Ans. A population of microorganisms routinely found in the normal healthy individual is known as normal flora of the body.
Q.11. Mention the advantages of normal flora.
Ans. Advantages of normal flora are:
Normal flora prevents colonization or invasion of the body by potential pathogens, either by simple competition with the pathogen for the nutrients or by producing inhibitory substances.
Some suitable examples are as under:
1. Vaginal Lactobacilli maintain an acidic environment by producing lactic acid, which suppresses growth of other microorganisms such as gonococci.
2. Fatty acids produced by skin bacteria prevent growth of other pathogens.
3. Some bacteria help in the disposal of waste matter.
4. Some bacteria present in the gut are able to produce vitamin B and vitamin K and thus may contribute in the nutrition.
5. The antigenic stimulation provided by the intestinal flora helps in increasing the immune status of the host.
Q.12. Name some media, which are commonly used, for the laboratory diagnosis of bacterial diseases.
Ans. Majority of the pathogens can grow on 5% sheep blood agar, which is the most widely used culture medium. Various selective media are used for cultivation of bacterial pathogens.
These include:
Q.13. Name the primary plating media used for following specimens:
Ans. (i) Urine (ii) CSF (iii) Pus
Following media will be used for primary plating of the specimens
(ii) Urine: Blood agar and Mac Conkey agar
(iii) CSF: Blood agar, chocolate agar and thioglycollate broth
(iv) Pus: Blood agar, chocolate agar and Mac Conkey agar
Q.14. What is botulism?
Ans. Botulism is a paralytic disease caused by neurotoxin of Clostridium botulinum, which is an obligately anaerobic endospore forming, Gram positive rod.
Q.15. Mention the names of pigments produced by Pseudomonas aeruginosa.
Ans. Two types of pigments are produced by Pseudomonas aeruginosa:
Pyoverdin – it is a fluorescent, water soluble greenish yellow pigment.
Pyocyanin – it is a non fluorescent phenazine blue pigment.
Q.16. What is plague?
Ans. The plague is a disease caused by Yersinia pestis (a small Gram negative rod), which infects rodents and is spread to humans either by the bite of rat flea (Xeopsylla cheopsis) or by inhalation of the droplets produced by the infected person. Two major forms of infection are bubonic plague and pneumonic plague:
Bubonic plague is characterized by high fever and painful inflammatory swelling of lymph nodes in the groin and the armpit. Pneumonic plague is caused by the inhalation of Yersinia pestis organism and is characterized by the malaise and pulmonary signs.
Q.17. Explain the Kirby-Bauer disk diffusion method for determining drug susceptibility.
Ans. The Kirby-Bauer disk diffusion test is routinely used to determine the susceptibility of a given microorganisms to a variety of chemotherapeutic agents. The isolate to be tested is inoculated over the entire surface of the Mueller Hinton agar plates and filter paper disks containing the wide variety of antimicrobial agents are applied over the surface of these agar plates.
After overnight incubation, the plates are examined for the presence of zone of inhibition around each antibiotic disk. The zone sizes are compared with established zone size ranges for individual antimicrobial agent and the results are recorded as ‘S’ (susceptible), ‘I’ (intermediate), or ‘R’ (resistant).
Q.18. What are coli-forms?
Ans. The coli-form group of bacteria is defined as all Gram negative non spore forming rods that are able to ferment lactose along with the production of acid and gas. These organisms typically reside in the intestinal tract of warm-blooded animals and thus serve as a bacterial indicator of warm-blooded animals.
Q.19. What are prions?
Ans. Prions are small proteinaceous infectious particles that contain only protein and no nucleic acid. They are able to cause a disease, which is known as spongiform encephalopathies.
The main characteristics of prions are:
1. Obligate intracellular parasites.
2. Smaller size (< 100 nm).
3. Lack of nucleic acid genome.
4. Resistant to inactivation by UV light or nucleases, but are susceptible to chemicals that denature proteins.
5. Slow replication.
6. Cannot be cultured in the laboratory.
Q.20. What are Negri bodies?
Ans. Negri bodies are prominent cytoplasmic inclusion bodies, which are found in the nerve cells of human beings and animals infected with rabies virus. Negri first described them in 1903. They are spherical or oval, 2µm – 20µm in diameter. Their characteristic appearance and easy recognition made possible the rapid diagnosis of rabies infection.
Q.21. What are the advantages and disadvantages of oral polio vaccine (OPV)?
Ans. The remarkable decline in poliomyelitis is due to the use of one or other of two polio vaccines currently available:
1. Killed virus vaccine
2. Attenuated live virus vaccine or oral polio vaccine (OPV)
The oral polio vaccine (OPV) has become the first choice in many countries. Its main advantages are:
3. Easy administration
4. Lower cost
5. Immunity is induced at right place, the mucosal surface
6. Confers “herd immunity” as well as individual immunity
7. Its effectiveness approaches 100%.
The disadvantages of OPV are:
1. Risk of reversion to increased virulence of the virus employed, particularly by type 2 and 3.
2. Dissemination of virus to unvaccinated contacts.
3. Can’t be used in immuno-compromised persons.
Q.22. Write a note on mode of action of antibacterial agents.
Ans. Several potential antimicrobial targets exist within the bacterial cell, but those pathways or structures frequently targeted include:
i. Inhibition of cell wall synthesis
ii. Damage to the cytoplasmic membrane
iii. Inhibition of nucleic acid
iv. Inhibition of protein synthesis
v. Inhibition of specific enzyme systems.
Summary of mechanism of action of commonly used antibacterial drugs:
