13.1 Microbiology Terminology and Specimen Appropriateness

CMLA Work Area III (Examination / Analytical Considerations) is 83 items, or 41.5% of the 200-question exam. Microbiology is competency 8. AMT III.8.A.1–2 requires terminology plus recognize appropriateness of specimen for type of microbiological test ordered. Named terms in this cluster include bacteria, aerobic, pathogenic, anaerobic, virus, yeast, fungal, culture, sensitivity, resistance, MIC, gram-negative rod, agar, incubator, inoculate, loop, gram-positive cocci, colony, normal flora, contaminant, A&P disk, alpha, beta, gamma hemolysis, candidiasis, exudates, organelles, agglutination, spores, Strep culture, AFB smear, KOH, Gram stain, wet prep, and culture and sensitivity (C&S). A Certified Medical Laboratory Assistant (CMLA) processes specimens, may inoculate plates only if the facility standard operating procedure (SOP) and lawful scope allow it, and prepares stains for someone else to read. You do not independently interpret high-complexity C&S or release antimicrobial susceptibility testing (AST).

Quick Answer: Bacteria are cells without membrane-bound organelles. Viruses are not cells. Yeast is a unicellular fungus. Aerobic needs oxygen; anaerobic does not. Pathogenic can cause disease; normal flora live on the body; a contaminant got into the specimen during collection. Gram-positive cocci are purple spheres; gram-negative rods are pink bacilli. Culture grows organisms on agar; C&S adds sensitivity. MIC is the lowest drug level that stops visible growth. Match the specimen to the order.

Bacteria, viruses, yeasts, fungi, organelles, and spores

Bacteria are microscopic prokaryotic organisms. They have cell walls (most of them), DNA, and ribosomes, but they lack membrane-bound organelles—no nucleus, no mitochondria. That is why “organelles” sits on the microbiology list: it is the contrast word. Yeasts and molds are fungal. They are eukaryotes; they do have organelles (nucleus, mitochondria). Candidiasis is infection with Candida, a yeast (thrush, vaginal yeast, some wound and blood isolates). A virus is acellular: nucleic acid in a protein coat, sometimes with an envelope. Viruses have no organelles and do not form colonies on ordinary agar. Viral detection is antigen, molecular, or specialized cell culture—not a blood-agar plate you sniff.

Spores appear in two teaching senses. Bacterial endospores (Clostridium, Bacillus) are dormant, heat- and chemical-resistant forms—the reason some spills need sporicidal disinfectant and the reason you do not treat every bench wipe as sterilization. Fungal spores are reproductive structures of molds. The stem tells you which kingdom is in play. Do not call a virus a spore-forming bacterium, and do not call candidiasis a viral plaque.

Aerobic, anaerobic, pathogenic, flora, and contaminant

Aerobic organisms grow in the presence of oxygen. Anaerobic organisms grow in the absence of oxygen; many are killed by room air (Section 13.2 transport). Facultative organisms can do both; CMLA must name the two official words, not invent a third as a waived kit.

Pathogenic means capable of causing disease. Normal flora (resident microbiota) are organisms expected at a body site: coagulase-negative staphylococci on skin, viridans streptococci in the throat, Escherichia coli in stool. Flora become a problem when they enter a normally sterile site (blood, spinal fluid, deep tissue) or when the specimen is collected so badly that flora look like infection.

A contaminant is an organism that entered the specimen during collection, transport, or processing—not a pathogen from the infection. Classic example: skin flora in a poorly prepped blood culture. The same Staphylococcus epidermidis can be flora on skin, a contaminant in a single blood-culture set, or a true pathogen on a prosthetic valve. CMLA does not make that call. You prevent contamination with aseptic collection and you label the site so the scientist is not guessing.

Exudates are inflammatory fluids—pus or drainage from a wound, abscess, or body cavity. An exudate in a sterile cup or anaerobic vial can be a valid culture specimen. The same pus smeared on a paper towel is not.

Gram-positive cocci, gram-negative rods, and the Gram stain idea

A Gram stain is a differential stain (steps in Section 13.4). Morphology plus color is how CMLA must talk:

• Gram-positive cocci — purple (crystal-violet retained) spheres. Staphylococci often cluster; streptococci and enterococci often chain or pair.
• Gram-negative rod — pink/red (safranin) bacilli. Enteric organisms and many other rods live here.

You prepare the smear. Qualified testing personnel typically read it. Do not report “GNR bacteremia” from a slide you glanced at, and do not call a yeast a gram-negative rod because it looked pink.

Culture, agar, inoculate, loop, incubator, colony

To inoculate is to transfer specimen or organism onto medium. A loop is the wire or disposable plastic tool used to streak or to deliver a calibrated volume (classic urine loops are 0.001 mL and 0.01 mL). Used wire loops are sharps when they can puncture (Section 13.2).

Agar is the solid (or semi-solid) culture medium in a plate or tube. Blood agar, MacConkey, chocolate, and specialized fungal or mycobacterial media are examples—not a chemistry gel barrier. A culture is the process of growing organisms in or on that medium. A colony is a visible mass that grew from one organism (or one clump). Counting colonies on a urine plate is how laboratories estimate colony-forming units; interpreting that count is not independent CMLA sign-out.

An incubator holds inoculated media at a controlled temperature, often about 35–37°C for routine bacteria, sometimes with carbon dioxide, sometimes in an anaerobic atmosphere. Fungi may use a different temperature. You verify the incubator temperature log (Chapter 4). You do not incubate a urine cup on a windowsill “to help it grow.”

C&S, sensitivity, resistance, and MIC

Culture and sensitivity (C&S) is two jobs: grow and identify what is there (culture), then test which antimicrobials inhibit it (sensitivity, also called susceptibility). Resistance means the organism is not inhibited at breakpoints the method uses—so that drug is a poor choice. MIC is minimum inhibitory concentration: the lowest concentration of an antimicrobial that prevents visible growth under standardized conditions.

AST and MIC methods are typically high complexity. A CMLA may log receipt, inoculate if SOP allows, and assemble reagents. You do not independently read zone sizes, assign S/I/R, or release an MIC panel. Confirmatory identification after a screen (direct fluorescent antibody or approved molecular testing, I.2.C) is the same idea: recognize the name, do not freelance the method.

Agglutination in microbiology is often a latex or particle kit that clumps when a specific antigen is present (streptococcal grouping, some Staphylococcus or Cryptococcus kits). Same lattice idea as Chapter 12, different department. Line darkness is still not a MIC.

Hemolysis and A&P disks

On blood agar, hemolysis describes what the colony does to red cells:

A&P disk means the A disk (bacitracin) and P disk (optochin) used with hemolytic streptococci. Classic teaching: Group A S. pyogenes is bacitracin-susceptible; S. pneumoniae is optochin-susceptible. Recognize the disks on a plate. Zone interpretation is scientist/technologist work, not a waived cassette a CMLA signs out.

Named preps: Strep culture, AFB smear, KOH, wet prep

Strep culture grows Group A streptococcus from a throat swab. It is the outline’s confirmatory path when a rapid Strep A screen is negative and the insert or policy requires culture or an approved molecular test (I.2.C). Collect a second swab into transport; do not wash a used antigen swab into the culture tube unless the instructions for use (IFU) validate that.

An AFB smear looks for acid-fast bacilli (mycobacteria, including Mycobacterium tuberculosis). Processing and staining may be allowed under SOP; independent AFB interpretation is not CMLA sign-out (Section 13.4).

KOH is potassium hydroxide. A KOH prep dissolves keratin so fungal elements are easier to see in skin, nail, or mucous-membrane scrapings. A wet prep (wet mount) is an unstained saline mount—motility, trichomonads, clue cells, yeast. Provider-performed microscopy (PPM) of some wet mounts is restricted to providers (Chapter 16). The assistant prepares; the assistant does not convert a wet prep into a PPM diagnosis.

Specimen appropriateness (III.8.A.2)

The right organism words still fail if the container is wrong.

Scenario

A provider orders “wound C&S, Strep culture, and sputum culture.” The collector sends one dry swab labeled “wound,” a nasopharyngeal swab “for strep,” and a cup of spit. None of those match. Recollect: site-labeled wound transport, a throat swab for Strep culture, and a true sputum—or reject with the reason documented.

Exam traps

• Calling viruses cells with organelles, or bacteria eukaryotic.
• Treating normal flora and contaminant as synonyms of pathogenic.
• Calling MIC a colony count or a waived chemistry level.
• Using an NP swab for Strep culture because “it is still the airway.”
• Independently releasing C&S or AST because you inoculated the plate.

Name the organism type, name the growth word, and name whether the specimen actually belongs to the test ordered.