D311 Microbiology - Set 1 - Part 3

Correct Answer: Remove crystal violet from thin-walled cells

Option a is correct because the decolorization step uses alcohol-acetone to wash crystal violet-iodine complexes out of gram-negative bacteria with thin peptidoglycan layers, while thick-walled gram-positive bacteria retain the purple dye, enabling differential classification. Option b is incorrect because applying the primary stain is the first step of gram staining where crystal violet dye is added to color all bacteria purple initially, occurring before the mordant and decolorization steps rather than being the function of the missed decolorization step. Option c is incorrect because fixing organisms to the slide is a preparatory step performed before staining begins, typically through heat fixation that kills and adheres bacteria to the glass surface, occurring well before the decolorization stage of the procedure. Option d is incorrect because adding the mordant compound is the second step where iodine is applied to form crystal violet-iodine complexes within all bacterial cells,

Correct Answer: Acid-fast stain

Option c is correct because the acid-fast stain uses heat to force carbolfuchsin dye into the mycolic acid-rich waxy cell walls of mycobacteria like Mycobacterium tuberculosis, which resist acid-alcohol decolorization while non-acid-fast organisms accept the blue counterstain. Option a is incorrect because the gram stain differentiates bacteria based on peptidoglycan thickness using crystal violet and safranin counterstain without requiring heat-driven dye penetration, and it cannot identify mycobacteria due to their unique waxy mycolic acid cell walls. Option b is incorrect because the endospore stain uses heat to drive malachite green dye into the resistant coat of dormant endospores produced by Bacillus and Clostridium species, not into the waxy mycolic acid walls of tuberculosis-causing mycobacteria. Option d is incorrect because the capsule stain is a negative staining technique that uses India ink or nigrosin to darken the background around encapsulated bacteria, revealing the cl

Correct Answer: Endospores

Option d is correct because the endospore stain uses steam-heated malachite green to penetrate the resistant keratin-like coat of dormant endospores, staining them green, while safranin counterstain colors surrounding vegetative bacterial cells pink for clear differentiation. Option a is incorrect because capsules are identified using negative staining techniques with India ink or nigrosin that darken the background to reveal clear halos around encapsulated cells, not through heat-driven malachite green penetration followed by safranin counterstaining. Option b is incorrect because flagella are typically visualized using specialized flagella staining protocols that apply mordants to thicken these thin appendages enough for light microscopy observation, not through malachite green and safranin differential staining with steam heating. Option c is incorrect because inclusion bodies are intracellular storage granules observed using specific stains like Sudan black for lipid inclusions or

Correct Answer: Differential staining

Option b is correct because differential staining techniques use multiple dyes applied in sequential steps to distinguish bacteria into separate categories based on structural differences such as cell wall composition, with the gram stain and acid-fast stain being primary examples. Option a is incorrect because negative staining uses a single acidic dye to color the background around organisms rather than staining the bacteria themselves, and it does not employ sequential multiple dyes to classify bacteria into different structural categories. Option c is incorrect because simple staining uses only a single dye to uniformly color all bacteria for basic visualization of size, shape, and arrangement, without the sequential multiple-dye approach needed to differentiate organisms based on structural characteristics. Option d is incorrect because fluorescent staining uses fluorescent dyes or antibodies that emit visible light under ultraviolet illumination for specific identification purpos

Correct Answer: Negative staining

Option a is correct because capsule staining is a negative staining technique that uses India ink or nigrosin to darken the background, leaving the capsule unstained and visible as a clear halo around each bacterium, confirming the presence of this antiphagocytic virulence factor. Option b is incorrect because acid-fast staining uses heat-driven carbolfuchsin to penetrate waxy mycolic acid cell walls of mycobacteria followed by acid-alcohol decolorization, not a background-darkening approach to reveal clear unstained halos around encapsulated bacterial cells. Option c is incorrect because gram staining is a differential technique using crystal violet and safranin to classify bacteria by peptidoglycan thickness into gram-positive and gram-negative categories, not a background-darkening method for visualizing capsular structures surrounding bacterial cells. Option d is incorrect because endospore staining uses steam-heated malachite green to penetrate resistant endospore coats followed b

Correct Answer: Median lethal dose

Option c is correct because the median lethal dose represents the number of pathogenic organisms or amount of toxin required to kill fifty percent of an infected test population within a specified time period, with lower values indicating greater virulence and lethality. Option a is incorrect because minimum inhibitory concentration measures the lowest concentration of an antimicrobial drug needed to prevent visible bacterial growth in laboratory culture, not the number of organisms required to cause death in fifty percent of test animals. Option b is incorrect because decimal reduction time measures the time required to kill ninety percent of a microbial population at a specific temperature during sterilization procedures, not the organism dose needed to produce lethal outcomes in half of test subjects. Option d is incorrect because the therapeutic index compares the toxic dose to the effective dose of a drug to evaluate its safety margin for clinical use, not the number of microorgan

Correct Answer: Primary pathogens

Option d is correct because primary pathogens possess inherent virulence factors powerful enough to cause disease in healthy individuals with intact immune systems, distinguishing them from organisms that require compromised host defenses or other predisposing conditions to establish infection. Option a is incorrect because opportunistic pathogens are organisms that typically cause disease only when host immune defenses are compromised by factors such as immunosuppression, antibiotic disruption of normal flora, or breached physical barriers, not in immunocompetent healthy individuals. Option b is incorrect because commensal organisms are members of the normal microbiota that coexist harmlessly with their host in a relationship where the microbe benefits without causing harm, rather than possessing virulence factors capable of causing disease in healthy individuals. Option c is incorrect because transient microbiota are temporary microbial residents that colonize body surfaces briefly w

Correct Answer: Median infectious dose

Option b is correct because the median infectious dose represents the number of pathogenic organisms required to successfully establish infection in fifty percent of an exposed population, with lower values indicating higher infectivity and greater ease of disease establishment. Option a is incorrect because minimum bactericidal concentration measures the lowest antimicrobial drug concentration needed to kill bacteria rather than inhibit their growth, representing a laboratory treatment metric rather than a measure of how many organisms cause infection. Option c is incorrect because thermal death point is the lowest temperature required to kill all microorganisms in a liquid sample within ten minutes during heat sterilization, not a measurement of the number of organisms needed to establish infection in exposed individuals. Option d is incorrect because zone of inhibition refers to the clear area surrounding an antibiotic disk on an agar plate where bacterial growth has been prevented

Correct Answer: Opportunistic pathogen

Option c is correct because opportunistic pathogens cause disease primarily in immunocompromised individuals whose defenses are weakened by factors like immunosuppressive medications, exploiting reduced immune function that would normally prevent their establishment and growth. Option a is incorrect because primary pathogens possess inherent virulence factors sufficient to cause disease in healthy individuals with fully intact immune systems, unlike organisms that require compromised host defenses from immunosuppressive medications to establish infection successfully. Option b is incorrect because commensal organisms coexist harmlessly with their host as part of the normal microbiota in mutually tolerant relationships, typically without causing disease even in immunocompromised patients, unlike the infection-causing organism described in this clinical scenario. Option d is incorrect because obligate parasites are organisms that require a living host to complete their life cycle and can

Correct Answer: Intrinsic bioremediation

Option c is correct because intrinsic bioremediation relies on naturally occurring indigenous microbial populations to degrade environmental contaminants without any human intervention or addition of external organisms, allowing native bacteria to metabolize pollutants through their inherent metabolic capabilities. Option a is incorrect because bioaugmentation involves deliberately introducing specific microorganisms to a contaminated site to enhance degradation of pollutants, which requires active human intervention rather than relying solely on naturally occurring indigenous bacterial populations already present. Option b is incorrect because pasteurization is a controlled heat treatment applied to foods and beverages to reduce pathogenic microorganism populations and extend shelf life, not a biological process where environmental bacteria naturally metabolize and break down petroleum pollutants. Option d is incorrect because fermentation is an anaerobic metabolic pathway that conver