3.3 Inoculation, Incubation Conditions, and Slide Preparation

Inoculation and incubation convert a correctly chosen plate into a readable culture. Outline I.B tests quantitative versus semiquantitative technique, automated plating, temperature, atmosphere, duration, and smear or wet-mount preparation. Isolation streaks create discrete colonies for identification; lawns create a confluent standard for antimicrobial susceptibility testing (AST). Mixing those two patterns is a common M(ASCP) trap.

Quantitative inoculation

Urine culture is the prototype quantitative setup. A calibrated 0.001 mL (1 µL) loop is dipped vertically into a well-mixed specimen and spread over the entire plate (or blood/MacConkey biplate). Each colony then represents 1,000 CFU/mL, so 100 colonies equal 100,000 CFU/mL, the traditional threshold for a significant midstream count in an uncomplicated adult. A 0.01 mL (10 µL) loop is more sensitive: each colony is 100 CFU/mL, which is preferred for pediatric patients, suprapubic aspirates, and some catheterized specimens where lower counts matter. Do not use a random uncalibrated loop and then multiply by 1,000.

When CFU/mL is the report

Quantitative respiratory culture, especially BAL, uses measured aliquots or calibrated loops so the laboratory can report CFU/mL against ventilator-associated pneumonia breakpoints (commonly 10^4 or 10^5 CFU/mL depending on specimen type and guideline). Tissue may be weighed and homogenized so a CFU/gram result can be calculated. Quantitative work is wasted if the specimen sat unrefrigerated and bacteria multiplied in the cup—the count then reflects delay, not infection. If the loop is bent, the volume is wrong and the math is fiction; replace the loop rather than “estimating.”

Semiquantitative streaking and automated platers

Most wound, sputum, and genital cultures are streaked in four quadrants: the first quadrant receives the swab; subsequent quadrants are streaked with a new line from the previous quadrant, flaming or using a new loop as policy requires. Growth in quadrant 1 only is reported as rare or 1+; growth through quadrant 4 is 4+ or heavy. That semiquantitative score helps the clinician distinguish a few colonies of colonizing yeast from a heavy predominant Staphylococcus aureus. It is not a CFU/mL number—do not convert 3+ into 10^5.

Automated plating instruments (WASP, InoqulA, and similar systems) pipet a defined volume and streak with a magnetic bead or loop, improving standardization of urine quantitation and reducing repetitive-motion injury. They do not replace judgment about rejection, Gram-stain correlation, or which media to load. If the instrument is down, the calibrated loop and four-quadrant streak must still be performed correctly by hand. An automated streak that looks like a lawn is the wrong pattern for a mixed wound culture that still needs isolated colonies.

Incubation temperature

Most medically important bacteria grow at 35–37°C. Fungi are often incubated at 30°C (some laboratories use 25–30°C for molds and a second 35–37°C plate for dimorphic conversion or yeasts from sterile sites). Campylobacter jejuni prefers 42°C, which also inhibits many competing stool flora. Yersinia enterocolitica can be recovered better with a room-temperature or cold-enrichment step in addition to 35°C. Do not incubate mycobacterial solid media at 42°C unless you are specifically looking for Mycobacterium xenopi; standard M. tuberculosis work is 35–37°C. Never assume “room temperature on the bench” is an incubator, and do not put a Campy-BAP in the CO2 incubator at 35°C and call it done.

Atmosphere

Ambient air (approximately 21% oxygen) suits Enterobacterales, Pseudomonas, and many yeasts. Capnophiles—Streptococcus pneumoniae, Haemophilus influenzae, Neisseria gonorrhoeae, N. meningitidis—require about 5% CO2 (a candle jar historically; a CO2 incubator now). Microaerophilic atmospheres (about 5% O2, 10% CO2, remainder N2) are mandatory for Campylobacter. Obligate anaerobes (Bacteroides, Clostridioides, Fusobacterium, anaerobic Gram-positive cocci) need anaerobic jars, pouches, or chambers with a palladium catalyst or gas-exchange system and an indicator (methylene blue or resazurin) that confirms anaerobiosis. Do not order anaerobic incubation for a sputum or voided urine plate. Chocolate and Thayer-Martin plates belong in CO2, not on the open shelf. If the anaerobic indicator shows that air leaked into the jar, the plates are not valid anaerobic cultures even if colonies grew.

Matching plate, gas, and temperature

The exam likes triple-match items: Campy-BAP at 42°C microaerophilic, chocolate at 35–37°C in CO2, MacConkey at 35–37°C ambient, LJ at 35–37°C for weeks, fungi at 30°C ambient (or as the mycology SOP specifies). If any one of the three conditions is wrong, the culture is wrong. A Haemophilus chocolate plate left in ambient air may fail to grow even though the medium itself was correct.

Duration

Routine bacterial plates are examined at 18–24 hours and held 48 hours; some sterile-site and anaerobic cultures are held 5 days. Continuous-monitoring blood-culture systems typically incubate 5 days for routine adult bottles (longer only when the laboratory’s protocol still extends for endocarditis or selected fastidious organisms). Mycobacteria on solid media are held 6–8 weeks; liquid MGIT flags earlier but negative liquids still complete a multi-week protocol (commonly 42 days). Fungal cultures are often held about 4 weeks (21–30 days depending on source). Do not discard a Legionella BCYE plate at 24 hours; Legionella may need 3–5 days. Campylobacter plates are read at 24 and 48 hours in the microaerophilic atmosphere. A 24-hour “no growth” on an anaerobic plate is not a final result.

Slide preparation

A Gram smear from a pellet or swab is spread thin, air-dried, and fixed with methanol or heat before crystal violet. Too thick a smear traps crystal violet and falsely looks Gram-positive; too much heat can distort morphology. Wet mounts (saline) show motility, trichomonads, fungal elements, and, when parasitology is in scope, stool trophozoites. KOH (10%) clears keratin and cellular debris so hyphae and yeast can be seen in skin, nails, and mucosal scrapings; calcofluor white can be added if a fluorescence microscope is available (covered more fully in the stain chapter). Do not heat-fix a KOH prep as if it were a Gram stain, and do not report a Gram stain from a dried, caked swab that should have been rejected.

Isolation streak versus lawn is an AST distinction. Identification needs isolated colonies from a four-quadrant streak. Disk diffusion and many gradient-diffusion tests need a confluent lawn inoculated from a 0.5 McFarland suspension; gaps in the lawn invalidate zone reading. Never read AST from a mixed primary plate. Automated susceptibility systems still start from isolated colonies, not from a swab of mixed 4+ growth. If the question shows a Mueller-Hinton plate with four quadrants and disks dropped on the first quadrant, the inoculation pattern is wrong even if the disks are the correct set.

If a question gives you a temperature, an atmosphere, and a plate name, match all three. That triple match is the inoculation-and-incubation item the exam writes.