8.4 Microbiology Setup and Plating
Key Takeaways
- The MLA supports microbiology with correct media selection patterns, sterile technique, labeling, and incubator/atmosphere handoff—not culture diagnostic sign-out
- Match common specimen types to basic media sets per SOP (e.g., urine vs wound vs respiratory patterns) without freelancing exotic workups
- Use sterile streaking technique when plating is in scope; label plates with patient identifiers, source, and date/time before or immediately at inoculation
- Load blood culture bottles and incubators per protocol (ambient vs CO₂); never report organism identification or susceptibility as a final diagnostic interpretation
8.4 Microbiology Setup and Plating
Quick Answer: In microbiology, the MLA sets up specimens—correct media, sterile technique, labeled plates/bottles, and proper incubator atmosphere—then hands work to the technical pathway defined by the lab. Learn common media patterns by specimen type. Do not interpret cultures, release organism IDs, or sign out susceptibilities as diagnostic results.
Microbiology is high-impact Domain II support work. Bad setup (wrong media, contaminated streak, mislabeled plate, delayed blood culture loading) can miss a pathogen or grow a contaminant that sends a patient down the wrong treatment path. Good setup is quiet patient safety.
Scope varies by state law, CLIA role, and employer. This section teaches exam-relevant support patterns. Always follow your facility’s competency assessments for who may plate, who may read plates, and who may enter results.
MLA Support Role vs Diagnostic Sign-Out
| Task | Typically in MLA support scope (when trained) | Not MLA diagnostic sign-out |
|---|---|---|
| Accession and label check | Yes | — |
| Select routine media per SOP chart | Yes | Designing novel multi-drug workups |
| Inoculate plates/broths with sterile technique | Often yes if competency documented | — |
| Load blood culture instruments | Yes | — |
| Incubator placement (O₂ vs CO₂ vs anaerobic jars) | Yes per SOP | — |
| Gram stain preparation (some labs) | Assist/prepare | Final morphologic diagnosis alone if not privileged |
| Colony identification & AST interpretation | — | No—licensed/credentialed micro tech/pathologist pathways |
| Critical micro result notification | May call under protocol after tech review | Independent clinical advice |
Exam mindset: If a question asks which organism “means” a disease or which drug to start, that is beyond MLA plate-setup knowledge. If it asks which media, temperature, atmosphere, or sterile step is correct, that is on-target.
Media Selection Basics by Specimen Type
Laboratories use SOP media charts. You are not inventing bacteriology—you are applying patterns.
| Specimen (common) | Typical media pattern (illustrative—follow local SOP) | Atmosphere notes |
|---|---|---|
| Clean-catch urine | Blood agar + selective gram-negative media (e.g., MacConkey); sometimes chromogenic urine agar | Ambient air incubator common for many urine protocols |
| Wound / abscess | Blood agar, MacConkey, and often anaerobic media if anaerobic culture ordered | CO₂ and/or anaerobic systems as ordered |
| Sputum / respiratory | Blood agar, chocolate, MacConkey (and special media if Legionella, etc., ordered) | CO₂ often needed for Haemophilus/fastidious growth on chocolate |
| Stool (bacterial culture) | Selective/differential enteric media set (e.g., MacConkey, XLD/HE, Campylobacter media as ordered) | Campylobacter needs microaerophilic conditions |
| Throat | Blood agar (and special for strep screens per method) | Ambient or CO₂ per method |
| Genital | Thayer-Martin/NYC-type media for Neisseria gonorrhoeae when ordered; other site media as charted | CO₂ critical for some pathogens |
| CSF / sterile body fluid | Enriched media (blood, chocolate) ± broth; rapid processing | CO₂ often; never delay |
| Blood culture | Instrumented bottles (aerobic ± anaerobic) per order—not agar first | Instrument incubation |
Teaching points, not full micro interpretation:
- Chocolate agar supports fastidious organisms that need extra factors.
- MacConkey selects for many gram-negative rods and differentiates lactose fermentation.
- Selective enteric media suppress normal stool flora to find Salmonella/Shigella-type pathogens.
- Anaerobic media/jars are used only when anaerobic culture is ordered and collected properly (not every swab).
- Viral, fungal, AFB, and molecular orders use different containers and setups—do not plate a viral swab on blood agar and call it complete.
If the specimen type is missing from the chart or the order is unclear, ask the microbiologist/tech—do not guess an elaborate media set.
Sterile Technique at Setup
- Disinfect the bench; gather only needed media.
- Check media expiration, cracks, contamination, and dehydration before use.
- Work near a flame only if your lab still uses that method; many modern labs use dead-air/BSC technique—follow local practice.
- Flame or use disposable loops correctly; one patient at a time when possible.
- Open plates briefly; do not leave stacks of open agar collecting room air.
- Do not touch agar with fingers, non-sterile pipette exteriors, or used swab shafts after they contact non-sterile surfaces.
- For quantitative urine cultures, use the calibrated loop volume specified (e.g., 0.001 mL or 0.01 mL)—wrong loop invalidates CFU/mL math.
- Cap broths and re-bag biohazard waste immediately.
Plating Streaks as an Assistive Task
When plating is in your scope, streaking isolates colonies and (for urine) supports quantitation.
Isolation streak (qualitative cultures)
- Inoculate a primary area from swab or pipette.
- Flame/change loop between quadrants if using classic four-quadrant method.
- Goal: isolated colonies in later quadrants—not a single paintbrush smear across the whole plate.
Quantitative urine streak
- Vertical then cross-streak patterns per SOP using a calibrated loop.
- Do not re-dip the loop into the specimen between streaks unless the method says so.
Poor streaks that produce only confluent growth make tech reading harder and may force subculture delays—still not an excuse for the MLA to “interpret” what grew.
Labeling Plates and Bottles
Label before inoculation or immediately in a controlled one-by-one process:
- Two patient identifiers
- Accession number
- Specimen source (urine, left leg wound, sputum)
- Date/time of inoculation when required
- Media type if not obvious from plate color/code
Write on the agar side skirt/bottom, not only the lid—lids swap. Never unlabeled “stack now, label later” during a rush.
Blood culture bottles: match bottle labels/barcodes to the patient and aerobic/anaerobic type; document draw volume when the system requires it.
Ambient vs CO₂ vs Other Incubation Handoff
After inoculation, atmosphere is part of the order’s success.
| Condition | Common use |
|---|---|
| Ambient air (~35–37 °C) | Many urine cultures, some general plates per SOP |
| CO₂ incubator (≈5% CO₂) | Chocolate agar, many respiratory and fastidious pathogens, some genital cultures |
| Anaerobic systems | Anaerobic culture orders only; proper collection critical |
| Microaerophilic | Campylobacter and similar |
| Room temperature / special | Some fungi or send-out holds—SOP driven |
| Blood culture instrument | Continuous monitoring bottles |
MLA handoff duties:
- Place plates in the correct incubator promptly.
- Do not leave inoculated plates on the bench overnight “to dry.”
- Do not put anaerobic plates in ambient air and hope.
- Log instrument loads if required; investigate bottle barcode errors immediately.
Blood Culture Bottle Loading Support
- Verify patient ID and that the correct bottle types were collected (aerobic/anaerobic/pediatric).
- Inspect for leakage, cracks, or grossly under/overfilled bottles if policy addresses fill.
- Clean bottle septa if residual blood contaminates the top (policy).
- Accession/load into the blood culture instrument without delay—time to incubation affects recovery.
- Do not vent or enter bottles except as directed by procedure (most modern systems are closed).
- When the instrument flags positive, follow the lab’s chain: notify tech, do not independently call a final organism ID from a Gram stain unless you are competency-assessed for that step and policy allows.
What You Must Not Do
- Diagnose infection from colony smell, color, or textbook memory and enter a final report.
- Change antibiotic orders based on preliminary growth.
- Discard plates early because “nothing important looks like it’s growing.”
- Mix two patients’ swabs on one plate to save media.
- Use expired or contaminated media to avoid walking to the refrigerator.
Workflow Summary
- Receive specimen; verify ID, source, and test orders.
- Consult media chart; gather in-date media.
- Label plates/bottles.
- Inoculate with sterile, specimen-appropriate technique.
- Incubate under correct atmosphere/temperature or load blood culture system.
- Document; hand off to technologist workflow for reading and reporting.
- Critical values and organism identification remain under authorized micro pathways.
Key Takeaways
- MLA micro work is setup excellence: media patterns, sterility, labels, atmosphere, blood culture loading.
- Learn common specimen–media–incubator patterns from the SOP chart—not freehand interpretation.
- Streak for isolation or quantitation when competent; label the plate bottom with full identity and source.
- Never sign out culture identification or susceptibility as a diagnostic interpretation outside authorized role.
Which task is outside the appropriate MLA role in microbiology?
A sputum culture is ordered. Chocolate agar is included in the SOP media set primarily because it:
When performing a quantitative urine culture, why must the MLA use the calibrated loop volume specified in the procedure?
Where should culture plate labels be written or affixed to reduce mix-ups?