Specimen Collection, Transport, and Primary Processing in Microbiology

Collection

Collect bacterial specimens before antimicrobial therapy begins, because even a single dose can suppress growth in culture. Viral specimens give the highest yield during the acute phase of illness, when shedding peaks. Specimen volume is as important as timing. Most methods have a minimum volume below which sensitivity falls sharply. Request recollection of an underfilled specimen, or ask the ordering clinician to prioritize the requested tests when the volume is insufficient for all of them.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Collect from the actual site of infection with as little contact with adjacent tissue and secretions as the site allows. Every specimen except stool goes into a sterile container. The label must include the patient's name or unique identifier and the specimen source.^{22 Laboratory requirements. 42 CFR part 493, §493.1242, §493.1256, §493.1291, §493.1445…} Laboratory procedure also requires the date and time of collection on the label, because every transport limit is measured from the collection time.^{1,31 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.3 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.} Whether an isolate is a pathogen or a contaminant depends on the body site the specimen came from and that site's resident flora. Urine container, labeling, and preservation practice is covered in Specimen Handling and Acceptability.

Swabs. A swab traps much of the collected material in its own fibers, so it delivers less specimen than an aspirate or a tissue sample. Use swabs for sites that cannot be sampled by aspiration or biopsy, such as the throat, the nasopharynx, or the cervix. Swabs are poor specimens for anaerobic, mycobacterial, and fungal culture. Send aspirated fluid or tissue for these cultures whenever the site permits.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Swab tip and shaft materials affect recovery. Some materials are toxic to the target organism, and others inhibit the amplification enzymes of a molecular assay:

Transport

Transport and acceptability limits depend on the requested test and collection system, so follow your laboratory's validated, test-specific instructions. The delayed-transport times below are examples. Your laboratory's instructions set its own rejection limits.

Transport temperature depends on the organisms expected at the collection site. Sterile-site fluids, blood, and gonococcal specimens may contain refrigeration-sensitive pathogens, so keep them at room temperature when delivery is delayed. Urine, respiratory secretions, and stool contain heavy resident flora that overgrows a pathogen at room temperature within hours, so refrigerate these specimens when delivery is delayed.^{1,31 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.3 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

Never refrigerate cerebrospinal fluid (CSF) for culture. Store and transport specimens collected into an FDA-cleared molecular collection kit as the kit instructions specify. These instructions commonly permit room temperature. The refrigeration rule above applies to culture specimens and viral transport medium.^{4,34 Papp JR, et al. Recommendations for the laboratory-based detection of Chlamydia… 2014.3 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

Molecular specimens. Nucleic acid testing does not require a viable organism, because lysed and dead cells still contain amplifiable target. It does require intact nucleic acid. DNA, and especially RNA, degrades quickly once cells lyse, so send molecular specimens promptly in a stabilizing medium. RNA targets require RNase-free handling of tubes, reagents, and work surfaces. Amplification can detect a single template molecule, so contamination control is as important as preventing degradation. Carryover from another specimen or a prior amplification can produce a false-positive result. When you suspect a control failure or a contamination event, stop releasing results and investigate before any result is reported.^{1,21 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.2 Laboratory requirements. 42 CFR part 493, §493.1242, §493.1256, §493.1291, §493.1445…}

This high sensitivity also affects interpretation. A positive molecular result can represent colonization, resolved infection, or an organism burden below clinical significance. Interpret the detection with the quantity and the clinical context. A primer set designed against one strain can miss a divergent variant of the same species. Collection timing must also match the stage of infection. In typhoid fever, blood culture is the preferred early specimen, although a single set detects only about half of cases. Stool culture is usually still negative during the first week of illness.^{77 Centers for Disease Control and Prevention. Typhoid and paratyphoid fever. 2026.}

The control-failure explanation gives the criteria for releasing patient results after a control failure.

Extraction of microbial nucleic acid follows the same principles as human-cell extraction, with four organism-specific complications:^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

1. Lysis resistance varies with cell-wall architecture. Mycobacteria have a lipid-rich, mycolic-acid-containing envelope, and fungal walls are built from structural polysaccharides such as glucans and chitin. Both resist lysis and need mechanical or chemical disruption, with an intensity matched to the organism and the validated assay. Gram-positive walls resist lysis more than gram-negative walls, and Mycoplasma species, which have no cell wall, lyse so easily that target can be lost to rough handling before extraction begins.
2. Concentration must match the reaction volume. Milliliter-scale fluids are centrifuged down to the microliter volumes used in extraction and amplification.
3. Amplification inhibitors must be removed. Hemoglobin and its breakdown products inhibit the polymerase in whole blood, and acidic polysaccharides inhibit it in sputum. Inhibitors in urine include urea, crystals, and hemoglobin.
4. RNA work adds RNase control at every step.

The preparation depends on the specimen type. Whole blood is processed to concentrate leukocytes, which harbor intracellular organisms and cell-associated viruses. Density-gradient separation is the classical method, and automated extraction platforms that remove hemoglobin now perform most routine extractions. Serum and plasma are the preferred sources for cell-free viral nucleic acid. Sputum needs inhibitor removal before extraction, and urine and CSF are centrifuged to pellet organisms.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Shipping to a reference laboratory. Referred specimens and cultures ship under the dangerous-goods rules as triple packages: a sealed primary receptacle; a leakproof secondary container with absorbent material sufficient to soak up the entire contents, with multiple primary tubes individually wrapped or separated; and a rigid outer shipping container. For Category B infectious substances (UN 3373) shipped by air, each primary receptacle is limited to 1 L and the outer package to 4 L. Category A packages have stricter limits, 50 mL or 50 g per package on a passenger aircraft and 4 L or 4 kg on a cargo aircraft, and add requirements such as an itemized contents list between the secondary and outer layers and taped, positively sealed closures.^{88 Transportation: hazardous materials… 49 CFR §172.101, §173.27, §173.196, §173.199 (2026).}

Receipt, prioritization, and rejection

Triage specimens by urgency, stability, and whether another specimen can be collected. When a compromised specimen cannot be replaced, contact the ordering service promptly and escalate through your laboratory's documented exception procedure. Resolve identity discrepancies before assigning a patient result. A clinician's request to proceed does not establish specimen identity. Document the acceptance decision and any limitation that must accompany the report. Recollect replaceable specimens that fail the applicable acceptance criteria.^{22 Laboratory requirements. 42 CFR part 493, §493.1242, §493.1256, §493.1291, §493.1445…}

Each laboratory director establishes the site's written acceptability and rejection criteria under the Clinical Laboratory Improvement Amendments (CLIA).^{22 Laboratory requirements. 42 CFR part 493, §493.1242, §493.1256, §493.1291, §493.1445…} The following rejections are standard for bacterial culture across clinical microbiology laboratories. Two entries are culture-specific and are marked as such, because the same specimen may be acceptable for a validated molecular assay or as part of a scheduled serial collection:^{1,31 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.3 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

Additional specimen types are rejected specifically for anaerobic culture because they are routinely contaminated with anaerobic resident flora that culture cannot distinguish from an anaerobic pathogen: gastric washings; urine other than a suprapubic aspirate; stool, except when anaerobic culture is specifically requested for outbreak typing or a suspected foodborne pathogen, since routine Clostridioides difficile testing uses toxin and nucleic acid assays; oropharyngeal specimens other than deep tissue collected at surgery; sputum; swabs of ileostomy or colostomy sites; and superficial skin specimens.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Worker protection

Treat every specimen as infectious. Organisms are inactivated at different points in collection, transport, and processing, and a processed specimen may still contain viable pathogens. Culture is the most exposure-prone step of a traditional workup, so molecular methods that test inactivated specimens directly are inherently safer.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.} Standard Precautions apply to blood; to all body fluids, secretions, and excretions except sweat, regardless of visible blood; to nonintact skin; and to mucous membranes. CDC created Standard Precautions in 1996 by unifying Universal Precautions, which addressed bloodborne pathogens, with Body Substance Isolation, which addressed organisms on moist body surfaces. The current statement is in the 2007 isolation-precautions guideline.^{99 Siegel JD, et al. 2007 guideline for isolation precautions. 2007.} The bloodborne pathogens standard of the Occupational Safety and Health Administration (OSHA) remains the enforceable federal regulation behind this practice and still uses the term universal precautions.^{1010 Occupational safety and health standards… 29 CFR §1910.1030, §1910.134 (2026). 1910.} Under CLIA the laboratory director may delegate safety duties to other staff and remains accountable for a safe laboratory environment.^{22 Laboratory requirements. 42 CFR part 493, §493.1242, §493.1256, §493.1291, §493.1445…} Handle a specimen suspected of containing a select agent, such as variola virus, only within the containment the agent requires, and refer it to the public health laboratory network. Maximum-containment work is restricted to biosafety level 4 facilities.^{1111 Centers for Disease Control and Prevention, et al. Biosafety in Microbiological and Biomedical Laboratories. 2020.}

Biological safety cabinet. The Class II Type A2 cabinet is the routine containment device in clinical microbiology. It protects the worker, the specimen, and the room. An inward air barrier at the sash, with a minimum face velocity of 100 linear feet per minute, sweeps room air into the cabinet and keeps aerosols out of the worker's breathing zone. Downflow air passed through a high-efficiency particulate air (HEPA) filter flows over the work surface and protects the specimen from room contamination. Exhaust air passes through a second HEPA filter before it returns to the room or leaves through a canopy connection. Recirculation to the room is acceptable only when no volatile toxic chemicals are used in the cabinet, because HEPA filters trap particulates and let vapors through. Use a chemical fume hood for work with volatile or flammable chemicals.^{1111 Centers for Disease Control and Prevention, et al. Biosafety in Microbiological and Biomedical Laboratories. 2020.}

Containment depends on work practices as much as on the cabinet. Move your arms in and out slowly so the air curtain stays intact. Keep the front and rear grilles clear of equipment, paper, and elbows. Work at least 4 inches inside the sash. Decontaminate the work surface before and after use and immediately after any spill, and run the cabinet for several minutes before and after work to purge airborne contaminants. Under the laboratory's accreditation requirements, each cabinet is certified to the NSF/ANSI 49 standard at installation, at least annually, after relocation, and after filter replacement or internal repair.^{1111 Centers for Disease Control and Prevention, et al. Biosafety in Microbiological and Biomedical Laboratories. 2020.}

Open and subculture any specimen or culture that may contain Mycobacterium tuberculosis, a mold, or a select agent inside a certified cabinet. Perform every aerosol-generating manipulation, including grinding tissue, vortexing, sonicating, and opening lyophilized cultures, inside the cabinet regardless of the suspected organism. The manipulation creates the aerosol hazard.^{1111 Centers for Disease Control and Prevention, et al. Biosafety in Microbiological and Biomedical Laboratories. 2020.}

Personal protective equipment and biosafety levels. Wear gloves, a fluid-resistant laboratory coat or gown, and face or eye protection for any procedure that could splash or aerosolize material. When an aerosol-generating procedure on a specimen likely to contain M. tuberculosis must be performed outside a biological safety cabinet, a fit-tested N95 respirator is the minimum respiratory protection. A powered air-purifying respirator (PAPR) gives a higher level of protection and is selected by risk assessment. Any respirator use falls under the OSHA respiratory protection program, which includes medical evaluation and training. Fit testing applies to tight-fitting respirators, including the N95 and tight-fitting PAPR facepieces. A loose-fitting PAPR hood or helmet does not require fit testing.^{10,1210 Occupational safety and health standards… 29 CFR §1910.1030, §1910.134 (2026). 1910.12 Centers for Disease Control and Prevention. Guidelines for preventing the transmission of Mycobacterium… 2005.} A respirator is the last control in the hierarchy and protects the worker only from aerosol that has escaped cabinet containment. Routine bacteriology is performed at biosafety level 2 (BSL-2). For M. tuberculosis, clinical specimens are processed at BSL-2 with every manipulation inside a biological safety cabinet, and propagation or manipulation of cultures requires BSL-3, which adds directional inward airflow and controlled laboratory access.^{1111 Centers for Disease Control and Prevention, et al. Biosafety in Microbiological and Biomedical Laboratories. 2020.}

Direct examination

The direct smear is the first examination of the specimen. It gives the clinician same-day information. It gives the laboratory a check on specimen quality and a guide to how much workup a recovered organism needs. An organism seen in abundance with many white blood cells warrants more identification work than the same organism seen sparsely with no inflammatory response. Recovery of the same organism from repeated specimens from one patient supports the significance of the isolate.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Gram stain. Spread an aliquot of the most purulent or bloody portion of the specimen to give a thick and a thin area, then air-dry and fix it. Methanol fixation is preferred because it preserves cell morphology and avoids the background distortion of heat. Gentle heat remains an acceptable alternative. Cytocentrifuge sterile fluids such as CSF first. Cytocentrifugation increases the yield of visible organisms by up to about 100-fold and can make a low organism burden detectable.^{1,131 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.13 Tunkel AR, et al. Practice guidelines for the management of bacterial meningitis. 2004.} The smear is stained with crystal violet, Gram's iodine, a decolorizer, and safranin. Gram-positive walls retain the purple crystal violet-iodine complex during decolorization. Gram-negative walls decolorize and take the safranin counterstain, pink to red.

Start at low power to find large structures such as nematode larvae, Curschmann spirals, sulfur granules, microcolonies, and fungal forms, then move to oil immersion at 1,000× to characterize bacteria. Roughly 10⁵ organisms per milliliter must be present for about one organism to appear per oil-immersion field, so a negative smear does not rule out a low-burden infection.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Describe size, shape, arrangement, and Gram reaction as specifically as the smear allows. A result of "gram-positive cocci in pairs, morphology consistent with S. pneumoniae" guides therapy far better than "gram-positive cocci." Quantify white blood cells and squamous epithelial cells, because they indicate inflammation and, in respiratory specimens, oropharyngeal contamination. The numeric adequacy thresholds for sputum are applied with the respiratory workup. Note intracellular organisms. Compare the smear with the culture. Organisms seen on the smear that fail to grow suggest an organism that needs special media, or a false-positive smear from contaminated reagents or misreading. For example, gram-negative coccobacilli that do not grow on blood agar suggest Haemophilus, and a chocolate plate on the next specimen can recover it. When one organism grows heavily in culture and was not seen on the smear, consider an organism burden below the smear's detection limit, sampling of a different part of the specimen, the smear's limited sensitivity, or contamination of the culture. Investigate each possibility.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Acridine orange. Acridine orange is a fluorochrome that intercalates into nucleic acid. In a smear buffered to about pH 4, bacterial and fungal nucleic acid fluoresces bright orange under blue or ultraviolet excitation. Human cell material fluoresces green. This color difference depends on the acidic staining conditions and is lost near neutral pH. Prepare and fix the smear as for a Gram stain, flood it with working solution, rinse, and read it promptly on a fluorescence microscope fitted with the correct excitation and barrier filters, because the fluorescence fades.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

The stain detects roughly 10⁴ organisms per milliliter, below the Gram stain's threshold. Its three classic uses are re-examining a Gram-stained smear with no organisms seen from a sterile site when infection is still suspected, screening blood culture broth for early growth before turbidity appears, and screening cell cultures for Mycoplasma, which lacks a cell wall and stains poorly by Gram. Its limitations follow from its chemistry. It stains all organisms alike and gives no Gram reaction. It stains nucleic acid in live and dead organisms, and background debris can fluoresce. Follow a positive result with a Gram stain or culture before it can guide therapy. It also requires a fluorescence microscope.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Media selection and inoculation

Media are chosen to support the pathogens expected at a site and to select them from the resident flora:^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Inoculation. When one specimen is inoculated to several media, inoculate plates before broth, and less selective media before more selective ones, so the richest media receive specimen before the inoculum is depleted. Then streak each plate for isolation. Apply the specimen to one edge and streak through three or four successive quadrants, sterilizing or turning the loop between passes. Each quadrant receives fewer organisms, until single cells are deposited far enough apart to grow into separate colonies. Conventional colony-based identification and susceptibility methods require isolated colonies. Validated direct methods from positive blood-culture broth have their own preparation, purity, and reporting requirements. For specimens in which the organism count affects interpretation, spread a fixed volume over the whole plate with a calibrated loop, commonly 0.001 or 0.01 mL. Multiply the colony count by 1,000 for a 0.001 mL loop and by 100 for a 0.01 mL loop to give colony-forming units (CFU) per milliliter.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Incubation

Routine bacterial cultures incubate at 35 ± 2 °C. An atmosphere of 5% to 10% CO₂ is essential or stimulatory for N. gonorrhoeae, H. influenzae, and S. pneumoniae, so use it whenever feasible.^{1,41 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.4 Papp JR, et al. Recommendations for the laboratory-based detection of Chlamydia… 2014.} Incubate selective and differential enteric media such as MacConkey, xylose-lysine-deoxycholate, and Hektoen enteric agar in ambient air. Added CO₂ acidifies the medium, which distorts the pH-indicator reactions used for differentiation and can reduce recovery of some gram-negative bacilli.^{1,141 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.14 Hasan MR, et al. Growth of clinically important gram-negative bacteria on… 2019.}

Anaerobic culture requires rapid transfer to an oxygen-free environment. The routine method is a sealed jar or pouch with a sachet that activates on exposure to air and absorbs oxygen while generating CO₂. Current sachet systems require no added water or catalyst. Older generator systems removed residual oxygen by reacting it with hydrogen over a palladium-coated alumina catalyst. High-volume laboratories use an anaerobic chamber, an airtight enclosure filled with an oxygen-free nitrogen-hydrogen-CO₂ mixture, entered through a gas-interchange lock, with glove or sleeve ports and an internal incubator. Palladium catalysis with the ambient hydrogen maintains anaerobiosis. Anaerobe recovery is comparable across these systems, which differ in cost, throughput, and convenience.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Routine plates are examined at 18 to 24 hours and generally incubated for 48 hours. Broths are incubated an additional 24 to 48 hours beyond their plates. Anaerobic plates are first opened at 48 hours, because the colonies grow slowly and early oxygen exposure reduces recovery. Anaerobic plates are incubated for at least 5 days. Organisms that need longer incubation have their own incubation times.^{11 Tille PM. Bailey & Scott's Diagnostic Microbiology. 2021.}

Reporting cadence

A preliminary report gives a verified finding before planned testing is complete. Its timing follows your laboratory's reporting procedure and the significance of the finding. CLIA does not require a preliminary report at every first examination. Label pending work and preliminary status clearly, so no reader mistakes a preliminary result for a final one. Findings on the laboratory's alert list, such as a verified positive blood-culture Gram stain, trigger the immediate notification pathway without waiting for final identification or susceptibility results.^{22 Laboratory requirements. 42 CFR part 493, §493.1242, §493.1256, §493.1291, §493.1445…}

The postanalytic reporting lesson explains staged reports, corrections, and documented confirmation that an urgent result reached its recipient. Before you plate a specimen, check the source, the collection time, and the container. Organisms lost or overgrown during collection and transport cannot be recovered at a later step.

Practice