Turnaround time for results
The speed of issuing a laboratory report directly depends on the biological characteristics of the microorganisms (their growth rate) and the chosen diagnostic method.
- Express information (1.5–2 hours). The earliest report the laboratory can provide after the biomaterial is received. It is based on Gram-stained smear microscopy. High informativeness requires a significant sample load — about $10^4–10^5$ CFU/mL. The report states only the microbial morphology (e.g., Gram-negative bacilli, Gram-positive cocci, molds, or yeast-like fungi). The main clinical significance of this stage is the ability to immediately prescribe empirical antibacterial therapy to the patient before the classic culture results are ready.
- Preliminary result (18–20 hours). Issued when a monoculture is present in the primary material at a high concentration. Specialists can apply antibiotic disks directly to the Petri dish with the primary culture.
- Final result. Includes complete species identification of the isolated strain and a detailed antibiogram. For aerobes and facultative anaerobes, this process takes at least 3–5 days. If strict anaerobes are involved, the waiting time increases to 6–8 days or more.
Quantitative evaluation and localization
When working with opportunistic microorganisms, it is critically important to evaluate their concentration (usually expressed in CFU/mL or CFU/g) with mandatory consideration of the type of locus examined.
There are loci and fluids that are normally completely sterile. These include blood, cerebrospinal fluid (CSF), urine (directly in the bladder), prostatic secretion, tears, saliva (as pure secretion of the salivary glands), and the respiratory tract below the level of the vocal cords. The detection of any bacteria in these areas is always of high diagnostic significance.
Resident microflora inhabits non-sterile loci. To assess pathology, it is necessary to know its normal concentration:
- Mucosa of the anterior nasal cavities: the concentration of a single species usually does not exceed $\le 10^2–10^3$ CFU/mL.
- Mucous membranes in general: the normal level of colonization is about $10^3$ CFU/mL.
- Feces: characterized by an extremely high concentration of bacteria — from $10^3$ to $10^{13}$ CFU/g.
How to distinguish the pathogen from contamination?
To establish whether the isolated strain is the cause of the disease or an accidental "guest" (a representative of normal flora or the result of sample contamination), a number of criteria for etiological significance are used.
Signs of a true infectious pathogen:
- A sharp increase in the concentration of one or two bacterial species is observed (pronounced dominance in the culture).
- The same microorganism species is isolated repeatedly during patient follow-up.
- The microorganism disappears from the locus as the patient clinically recovers.
- A completely identical species is isolated in a whole group of patients with a similar clinical presentation (typical for epidemic outbreaks).
Signs of sample contamination:
- The laboratory detects associations of three or more different microbial species.
- Bacteria are detected in very low concentrations.
- Representatives of normal microflora whose presence is completely uncharacteristic of the pathological process in this specific locus are isolated.
Quality of the pre-analytical phase
The reliability of a microbiological report depends not only on laboratory equipment, but also on strict adherence to the rules for collecting and transporting biomaterial. In case of clear signs of pre-analytical violations, the laboratory has the right to reject the material, as the study will be impractical.
- Sputum collection error: If during initial smear microscopy the laboratory physician sees a large number of buccal squamous epithelial cells while leukocytes are absent, it is concluded that saliva entered the container. Such a sample does not reflect the actual condition of the patient's lower respiratory tract.
- Urine collection error: If a urinalysis detects several bacterial species simultaneously at a low titer ($< 10^3$ CFU/mL), this indicates mundane sample contamination by microflora washed from the mucosa of the external genitalia. Conducting a bacteriological study of such a sample is pointless.
- Bile collection features: The most reliable material is bile obtained intraoperatively. If the biomaterial is obtained via duodenal probing, there is a high risk of its contamination by the microflora of the upper gastrointestinal tract and oropharynx during the probe's passage. It is worth noting that the isolation of a single microbial species from bile at a high titer may indicate a serious pathology, such as a subdiaphragmatic or liver abscess.
Dynamic monitoring and epidemiological markers
Microbiological testing is rarely limited to a single culture. For an objective assessment, dynamic monitoring is required. Serial cultures are performed 3–5 times, and in an inpatient setting, repeated every 7–10 days.
Repeated tests make it possible to clarify the etiology (confirm the initial diagnosis), assess the duration of pathogen persistence in the body, and monitor therapy effectiveness by comparing microbial sensitivity in vitro with the actual clinical effect.
Epidemiological markers are used to identify the source of a healthcare-associated infection and transmission routes. This consists of intraspecies typing of isolated strains. To identify a specific hospital strain, its specific markers are determined:
- Antibiovars (grouped by antibiotic resistance profile).
- Phagovars (grouped by susceptibility to specific bacteriophages).
- As well as biovars, chemovars, and serovars.
Briefly: answers to topical questions
What methods are used for complete species identification of bacteria at the final reporting stage?
The bacteriological (cultural) method is primary for complete species identification of bacteria. Identification of the isolated pure culture is carried out based on a complex of the following properties:
- Morphological and tintorial — evaluation of the shape and staining of microorganisms.
- Cultural — character of growth on nutrient media.
- Biochemical — the basis of species classification (evaluation of carbohydrate and protein metabolism, etc.).
- Toxigenic and antigenic — detection of specific toxins and antigens.
The final result is issued after culture isolation and includes complete identification of the pathogen.
Which microorganisms most frequently act as contaminants in blood cultures?
Contaminants in blood cultures can be representatives of obligate skin microflora, in particular coagulase-negative staphylococci (predominantly Staphylococcus epidermidis) and Corynebacterium spp. If these microorganisms are detected in only one of two blood culture bottles, the result is considered sample contamination. To confirm their clinical significance, two positive blood cultures are required.
The Rule of Three D's: the true pathogen Dominates (at a high concentration), persists in Dynamics (in serial cultures), and is Distributed among others (isolated from a group of individuals during an outbreak).
Class and exam questions
In which cases can the laboratory issue a preliminary result within 18–20 hours?
This is possible if the biological material contains a monoculture of a microorganism with a very high bacterial load. In such situations, the laboratory technician can place antibiotic disks directly onto the primary isolation plate.
How to determine from primary microscopy that the patient provided saliva instead of sputum?
In a poor-quality sample, a large number of buccal epithelial cells will be detected under the microscope in the complete absence of leukocytes. The result of such a culture will not reflect the state of the lower respiratory tract.
Why does bile obtained during duodenal probing often yield unreliable results?
During the insertion of a duodenal probe, inevitable contamination of the sample with the normal microflora of the oropharynx and upper GI tract occurs. To obtain the most reliable data, bile should be collected intraoperatively.
What else to study on this topic
- Features of cultivation and identification of strict anaerobes
- Intraspecies typing methods: phages typing and serotyping
- Rules for collecting biological material from non-sterile sites (throat, nose, wounds)
- Correlation between in vitro microbial susceptibility and clinical efficacy
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