Clinical Microbiology Test #5 Study Guide

Comprehensive practice flashcards covering bacteriology, susceptibility testing, equipment, mycology, parasitology, and virology from the Clinical Microbiology Test #5 Study Guide.

What is the formula used to calculate urine culture colony counts in $\text{CFU/mL}$?

$$\text{Number of colonies counted} \times \text{Loop dilution factor} = \text{CFU/mL}$$

How is the colony count calculated when using a $0.01\,\text{mL}$ calibrated inoculating loop?

Multiply the number of colonies counted by $100$ (e.g., $45\text{ colonies} \times 100 = 4{,}500\,\text{CFU/mL}$).

How is the colony count calculated when using a $0.001\,\text{mL}$ calibrated inoculating loop?

Multiply the number of colonies counted by $1{,}000$ (e.g., $75\text{ colonies} \times 1{,}000 = 75{,}000\,\text{CFU/mL}$).

What is microbiology?

The study of microorganisms (microbes)—living organisms too small to be seen with the naked eye, including bacteria, viruses, fungi, protozoa, and helminths.

What are the proper nomenclature rules for writing bacterial and parasitic names?

Names must be italicized when typed (or underlined separately when handwritten), written with the Genus capitalized and species lowercase (e.g., Staphylococcus aureus or S. aureus after first mention).

What are the cellular characteristics and reproduction method of bacteria?

They are single-celled (unicellular) prokaryotic microorganisms that multiply asexually by binary fission, where one cell divides into two identical daughter cells.

What is the distinction between pathogens, opportunistic pathogens, and normal flora?

Pathogens cause disease in a host; opportunistic pathogens cause disease only when the host's immune system is impaired or when introduced into a sterile body site; normal flora (microbiota) live on or in the body without causing harm and often benefit the host.

How do obligate aerobes, obligate anaerobes, facultative anaerobes, and microaerophiles differ in their oxygen requirements?

Obligate aerobes require $O_2$; obligate anaerobes grow only in the absence of $O_2$ ($O_2$ is toxic); facultative anaerobes grow with or without $O_2$; and microaerophiles require reduced oxygen levels (typically $\sim 5\%\,O_2$).

What is the optimal incubation temperature for culturing most human pathogenic bacteria?

$35^\circ\text{C}$ to $37^\circ\text{C}$ (normal human body temperature).

What is the difference between an antiseptic and a disinfectant?

An antiseptic is a chemical agent safe to apply to living tissue to destroy or inhibit microbes (e.g., alcohol prep pads, iodine, chlorhexidine), whereas a disinfectant is a stronger chemical agent used on inanimate surfaces (e.g., $10\%$ bleach solution) that is too harsh for living tissue.

Which biosafety levels handle routine clinical microbiology specimens versus the most dangerous, exotic pathogens?

Routine clinical microbiology laboratories operate at BSL-2 (moderate-risk agents like Staphylococcus and Salmonella), while BSL-4 represents the highest containment level for dangerous, life-threatening aerosol-transmitted agents without vaccines or cures (such as Ebola virus).

What is the preferred specimen type for a wound culture, and why?

A tissue biopsy or fluid aspirate collected via needle and syringe, because swabs pick up superficial skin flora and hold minimal sample volume.

What is the function of transport media in microbiological specimen handling?

It preserves specimen viability and maintains moisture and a neutral pH while preventing the overgrowth of fast-growing contaminants during transit; it contains no nutrients for multiplication.

What are common criteria for rejecting a microbiology specimen?

Unlabeled or mislabeled containers, leaking/broken containers, delayed transport (>$2\,\text{hours}$ without transport media or refrigeration), dried-out specimens, specimens collected in formalin/preservative, duplicate specimens within $24\,\text{hours}$, or inappropriate specimen types for the test requested.

What is the primary function of a Bacti-Incinerator in a microbiology laboratory?

It uses infrared heat inside a ceramic core to safely sterilize metal inoculating loops and needles without open flames or aerosol splatter.

What method is used to obtain pure, isolated bacterial colonies on an agar plate?

The four-quadrant streak plate method, which sequentially dilutes bacteria across four quadrants until individual cells separate to form pure colonies.

How do primary (enriched), selective, and differential growth media differ?

Primary media (e.g., Blood agar) support most non-fastidious bacteria; selective media (e.g., MacConkey, EMB) contain inhibitors to suppress unwanted bacteria while selecting for specific organisms; differential media contain indicators that distinguish organisms based on biochemical reactions or colony appearance.

What are the three types of hemolysis observed on Blood Agar Plates (BAP)?

Alpha ($\alpha$) hemolysis (partial breakdown of RBCs with a greenish/brownish halo), Beta ($\beta$) hemolysis (complete RBC lysis forming a clear zone), and Gamma ($\gamma$) hemolysis (no breakdown or change in agar).

Why must agar plates be incubated upside down (agar side up / lid down)?

To prevent condensation from dripping onto the agar surface, which would cause bacterial colonies to run together.

What is the essential first step before setting up bacterial identification tests?

Determining the Gram reaction (Gram-positive vs. Gram-negative) and cellular morphology.

What color do Gram-positive and Gram-negative bacteria appear after staining, and why?

Gram-positive bacteria stain purple/dark blue because their thick peptidoglycan wall retains the crystal violet-iodine complex; Gram-negative bacteria stain pink/red because their thin peptidoglycan and outer lipid membrane decolorize and take up safranin.

What is the correct sequence of reagents used in the Gram staining procedure?

1. Crystal violet (primary stain) $\rightarrow$ 2. Gram's iodine (mordant) $\rightarrow$ 3. Decolorizer (alcohol/acetone) $\rightarrow$ 4. Safranin (counterstain).

What is the purpose of heat-fixing a slide, and what risk is associated with overheating?

Heat-fixing adheres bacteria to the glass slide and kills them, but excessive heat can distort or destroy microbial morphology.

How are Staphylococcus and Streptococcus differentiated by test results and microscopic appearance?

Staphylococcus species are catalase-positive (produce bubbles) and appear as Gram-positive cocci in clusters; Streptococcus species are catalase-negative (no bubbles) and appear as Gram-positive cocci in chains.

Which test differentiates Staphylococcus aureus from coagulase-negative staphylococci (CoNS)?

The coagulase test: Staphylococcus aureus is coagulase-positive (forms a fibrin clot), whereas other Staphylococcus spp. (e.g., S. epidermidis) are coagulase-negative.

How can Escherichia coli and Klebsiella spp. be distinguished on EMB and MacConkey agar?

Escherichia coli produces a distinctive green metallic sheen on EMB agar and bright pink colonies on MacConkey; Klebsiella spp. produce large, wet, mucoid pink colonies on both agars due to a thick capsule.

Why is touching red blood cells on blood agar during a catalase test problematic?

Red blood cells naturally contain catalase, so accidentally picking up agar with the colony can cause a false-positive bubbling reaction.

How is a Kirby-Bauer susceptibility plate inoculated and measured?

It is inoculated with a sterile swab streaked in three directions to make a confluent bacterial lawn; the resulting zones of inhibition are measured across the diameter in millimeters ($\text{mm}$).

What is the Minimum Inhibitory Concentration (MIC)?

The lowest concentration of an antimicrobial agent (in $\mu\text{g/mL}$) that completely inhibits visible growth of a microorganism after overnight incubation.

What molecular mechanism causes methicillin resistance in MRSA (Methicillin-Resistant Staphylococcus aureus)?

The mecA gene encodes an altered penicillin-binding protein (PBP2a) that prevents beta-lactam antibiotics from binding.

What are the key identification characteristics of Group A Streptococcus (Streptococcus pyogenes)?

Gram-positive cocci in chains, catalase-negative, beta-hemolytic, Lancefield group A, and susceptible to bacitracin (sensitive to the A-disk).

What anatomical sites must be sampled—and which avoided—when collecting a throat culture for Group A Strep?

Touch the swab only to the tonsils and posterior pharynx; avoid touching the tongue, cheeks, and uvula.

What is the most common Gram-negative and a notable Gram-positive cause of urinary tract infections?

Escherichia coli is the most common overall cause (Gram-negative rod); Staphylococcus saprophyticus is a notable Gram-positive cause, especially in young sexually active females.

What is the reporting protocol for a urine culture growing three or more distinct organism types?

Report the culture as 'mixed' (indicating probable contamination) and do not provide individual colony counts.

How do yeasts and molds differ regarding cellularity, optimal temperature, and reproduction?

Yeasts are unicellular, grow at $37^\circ\text{C}$, and reproduce by budding (blastoconidia); molds are multicellular filamentous fungi forming hyphae/mycelia, grow at $25^\circ\text{C}$, and reproduce via spores (conidia).

What is a dimorphic fungus?

A fungus that exists in two morphological forms depending on temperature: mold at room temperature ($25^\circ\text{C}$) and yeast at body temperature ($37^\circ\text{C}$) ('mold in the cold, yeast in the heat').

What is the scientific name of the pinworm, and how is it diagnosed?

Enterobius vermicularis; diagnosed using a Scotch tape preparation applied to the perianal area to collect ova.

Which parasite is frequently identified in urine wet mounts by its jerky motility?

Trichomonas vaginalis, a flagellated protozoan.

What diagnostic morphology indicates Plasmodium sp. infection on a peripheral blood smear?

Characteristic signet-ring trophozoite forms located inside red blood cells (RBCs).

What is the relative size order and approximate size ranges of viruses, bacteria, and human cells?

Viruses ($\sim 20\text{--}300\,\text{nm}$) < Bacteria ($\sim 1\text{--}5\,\mu\text{m}$) < Human cells ($\sim 10\text{--}100\,\mu\text{m}$).

What are the two basic structural components found in all viruses?

A nucleic acid core (consisting of either DNA or RNA, never both) and a surrounding protein coat (capsid).