Collection rules and transportation features of the material
The reliability of the microbiological examination of cerebrospinal fluid directly depends on the quality of the pre-analytical phase. The sample is collected by a physician via lumbar puncture or puncture of the lateral cerebral ventricles. The procedure requires strict adherence to aseptic techniques to prevent contamination. A 1–2 ml volume of the obtained CSF is collected into a strictly sterile tube.
A critical factor for successful diagnostics is proper transportation. The main hazard at this stage is sample cooling. A number of pathogens, particularly Neisseria meningitidis (meningococcus), exhibit high sensitivity to low temperatures and undergo rapid death when the tube cools down.
For this reason, the material must be delivered to the laboratory within 15–20 minutes while the CSF remains warm. Special equipment is used for transportation: thermoses, heating pads, or isothermal boxes capable of maintaining a temperature of 37 °C. If immediate delivery is impossible for any reason, the sample must be stored in an incubator at 37 °C (storage is permissible for several hours).
Laboratory processing and direct microscopy
The first stage of laboratory analysis is the preparation of a smear for microscopic examination.
The standard preparation algorithm includes mandatory concentration of cells and microorganisms. To achieve this, the tube with CSF is centrifuged for 5 minutes at 3,500 rpm. After centrifugation, the supernatant is discarded, and a smear is prepared from the resulting sediment.
However, there is an important exception to this rule. If the received cerebrospinal fluid is visibly turbid or contains a characteristic pellicle, the smear is prepared immediately, without prior centrifugation.
Microscopy has high diagnostic value, especially in purulent meningitis. The method allows for the rapid detection of tuberculosis pathogens, meningococci, and opportunistic flora, providing the physician with valuable information even before the complete bacteriological analysis is finished.
Culture examination (CSF culture)
To isolate a pure culture of the pathogen, inoculation is performed on enriched nutrient media. The standard set includes 5% blood agar, serum agar, and thioglycollate broth. The inoculation technique involves applying 2–4 drops of native CSF or pre-resuspended sediment onto the surface of the medium and thoroughly spreading it with a loop.
Incubation of the cultures is carried out at 37 °C using different gas atmospheres to cover the maximum spectrum of potential pathogens:
- Plate 1: incubated under normal atmospheric conditions.
- Plate 2: incubated in an atmosphere with an increased carbon dioxide ($CO_2$) concentration.
The use of carbon dioxide is strictly justified: $CO_2$ is a potent growth stimulator for microorganisms such as pathogenic neisseriae, pneumococci, and listeriae.
When specific infections are suspected, additional media are used. For example, if the clinical presentation indicates Haemophilus influenzae infection, inoculation must be performed on "chocolate" agar. Once visible colony growth appears, microbiologists proceed to the identification phase of the isolated microorganism.
Algorithm completion and result interpretation
The final and most critical clinical stage of the analysis is determining the antibiotic susceptibility of the isolated microbial culture. If the initial culture yields no growth (negative result), the protocol requires a repeat culture after 8–10 days.
The interpretation of results relies on the fact that cerebrospinal fluid is normally sterile. Therefore, the isolation of any microorganism while following collection protocols is considered pathological, and the microbe itself is recognized as the etiological factor of the disease.
A special approach is required when diagnosing secondary meningitis. If meningeal inflammation develops as a complication of existing purulent processes (such as otitis media, sinusitis, phlegmon, or abscess), the clinician must ensure microbiological examination of the primary source of infection. The main goal of this procedure is to detect and match identical strains of the pathogen in the primary purulent focus and in the CSF.
Briefly: answers to topical questions
Which staining methods are used for microscopic examination of cerebrospinal fluid?
Microscopic examination of cerebrospinal fluid material utilizes:
- Gram staining;
- Gram-Weigert staining;
- Romanowsky-Giemsa staining;
- PAS reaction;
- acridine orange staining;
- for CSF sediment microscopy — an express method with a drop of India ink to detect encapsulated budding yeast cells.
Which pathogens can be detected by direct microscopy of a cerebrospinal fluid smear?
Direct microscopy of a cerebrospinal fluid smear can detect bacterial and fungal pathogens.
- Meningococci (Neisseria meningitidis) — visualized as Gram-negative bean-shaped diplococci, located intracellularly and extracellularly.
- Tuberculosis pathogens — detected using specific staining.
- Opportunistic flora — found in purulent meningitis.
- Yeast fungi (Cryptococcus spp.) — identified as encapsulated budding cells against a dark ink background.
- Yeast-like cells — detected intracellularly within histiocytes and extracellularly in endemic mycoses (double-contoured, oval, with a single bud).
Which bacteria are the primary causative agents of primary purulent meningitis?
The primary causative agent of primary purulent meningitis is meningococcus.
- Meningococcus (Neisseria meningitidis) — a Gram-negative diplococcus causing meningococcal meningitis. This microorganism is responsible for over 50% of primary purulent meningitis cases in children over 2 years of age.
Other bacteria, such as pneumococcus (Streptococcus pneumoniae), more commonly cause secondary purulent meningitis as a complication of otitis media or sinusitis. Haemophilus influenzae (Haemophilus influenzae) is the most frequent cause of purulent meningitis in children, but in classification it is distinguished separately from primary meningococcal infection.
What changes in biochemical CSF parameters are characteristic of purulent bacterial meningitis?
Purulent bacterial meningitis is characterized by the following CSF changes:
| Parameter | Changes |
|---|---|
| Protein | Elevated: up to 1.5–6 g/L or more; the source also cites a range of 0.66–16.0 g/L. |
| Glucose | Decreased: less than 1.5 mmol/L or less than 40 mg/dL. |
| Lactate | Elevated: up to 4.1–22.3 mmol/L. |
| pH | Decreased. |
| Fibrin D-dimer | Elevated: up to 736–3110 ng/mL. |
| Pandy's test | Strongly positive (+++). |
Neutrophilic pleocytosis and cell-protein dissociation are also characteristic.
The rule of three "T"s will help remember the critical conditions for working with CSF: Timely (asepsis), Temperature (37 °C), and Tempo (delivery within 15 minutes).
Class and exam questions
Why must CSF be delivered to the laboratory so quickly (within 15–20 minutes)?
This is due to the high sensitivity of certain pathogens to cooling. For example, meningococcus (N. meningitidis) rapidly dies when the temperature drops, and prolonged delivery may result in a false-negative culture.
What should be done if rapid transport of cerebrospinal fluid is not possible?
In such cases, the tube must be strictly stored in an incubator at 37 °C. Under these conditions, the material can remain suitable for analysis for several hours.
Why is one of the Petri dishes incubated in a CO2-enriched atmosphere during CSF inoculation?
An increased carbon dioxide concentration is necessary to stimulate the growth of certain fastidious bacteria. These include pathogenic Neisseria, pneumococci, and Listeria.
In which case is a CSF smear for microscopy prepared without prior centrifugation?
Centrifugation can be omitted if the received cerebrospinal fluid is already visibly turbid or if a pellicle has formed on its surface. Under normal conditions, however, CSF is centrifuged to concentrate the causative agent in the sediment.
What else to study on this topic
- Technique and guidelines for performing a lumbar puncture.
- Determination of the susceptibility of the isolated culture to antibiotics.
- Characteristics of inoculation on chocolate agar for H. influenzae.
- Microbiological examination of primary purulent foci in secondary meningitis.
- Morphology and cultural characteristics of Neisseria meningitidis.
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