Laboratory Contaminants

Table of Contents

Common bacteriology Laboratory Contaminants

In bacteriology labs, it is crucial to differentiate between true pathogens and common contaminants.
Contaminants are microorganisms that are not the cause of disease but can be introduced into specimens unintentionally.
Below are more detailed descriptions of common bacterial contaminants:

Skin Flora Contaminants

These bacteria are part of the normal human skin flora and are often introduced during specimen collection, particularly blood cultures or skin swabs.

- Staphylococcus epidermidis:
  - Characteristics: Coagulase-negative, gram-positive cocci in clusters.
  - Clinical Significance: While it is typically non-pathogenic, S. epidermidis can cause infections in immunocompromised patients or those with indwelling devices.
  - Laboratory Diagnosis: Catalase positive, coagulase negative, grows on blood agar.
- Cutibacterium acnes (formerly Propionibacterium acnes):
  - Characteristics: Gram-positive, anaerobic, or facultative anaerobic, pleomorphic rods.
  - Clinical Significance: It is usually non-pathogenic but can be associated with acne, prosthetic device infections, or post-surgical wound infections.
  - Laboratory Diagnosis: Catalase positive, non-motile, and produces small, white colonies on blood agar.
- Corynebacterium species:
  - Characteristics: Gram-positive, non-spore-forming rods.
  - Clinical Significance: Often part of normal flora, but some species like Corynebacterium diphtheriae can be pathogenic.
  - Laboratory Diagnosis: Grows on selective media like Tinsdale agar for identification.

Environmental Contaminants

These bacteria are frequently found in the environment, especially in soil, water, and surfaces.

- Bacillus species:
  - Characteristics: Gram-positive, spore-forming rods.
  - Clinical Significance: Bacillus cereus can cause food poisoning or wound infections, but other species like Bacillus subtilis are usually contaminants.
  - Laboratory Diagnosis: Large, round, white or off-white colonies, beta-hemolytic on blood agar.
- Pseudomonas species:
  - Characteristics: Gram-negative, oxidase-positive, non-fermentative rods.
  - Clinical Significance: Pseudomonas aeruginosa is a pathogenic species, but non-pathogenic species (e.g., Pseudomonas putida) may occasionally contaminate cultures.
  - Laboratory Diagnosis: Greenish pigment production (pyocyanin), fruity odor, oxidase positive.
- Acinetobacter species:
  - Characteristics: Gram-negative coccobacilli, non-fermentative.
  - Clinical Significance: Acinetobacter baumannii is pathogenic, especially in hospital settings, but other species like Acinetobacter lwoffii may be contaminants.
  - Laboratory Diagnosis: Growth on MacConkey agar, non-lactose fermenter, and oxidase negative.

Water-Associated Contaminants

Some bacteria thrive in water and may contaminate laboratory cultures if water or equipment is disinfected.

- Mycobacterium gordonae:
  - Characteristics: Acid-fast, slow-growing, non-pathogenic mycobacterium.
  - Clinical Significance: Often found in tap water, it is non-pathogenic but can be mistaken for Mycobacterium tuberculosis.
  - Laboratory Diagnosis: Slow-growing, acid-fast on Ziehl-Neelsen staining, non-chromogenic.
- Stenotrophomonas maltophilia:
  - Characteristics: Gram-negative, oxidase-negative, non-fermentative rod.
  - Clinical Significance: Can cause opportunistic infections, especially in immunocompromised patients, but often a contaminant in laboratory cultures.
  - Laboratory Diagnosis: Lavender-green colonies on MacConkey agar, oxidase negative.

Laboratory-Associated Contaminants

These contaminants are typically introduced through improper sterilization of materials or accidental handling.

- Enterobacteriaceae species (e.g., Escherichia coli, Klebsiella):
  - Characteristics: Gram-negative, facultatively anaerobic, lactose-fermenting rods.
  - Clinical Significance: Some species are pathogenic (e.g., E. coli), but non-pathogenic strains are often laboratory contaminants.
  - Laboratory Diagnosis: Lactose fermentation on MacConkey agar, indole positive for E. coli.

Common Fungal Contaminants in the Laboratory

Fungal contaminants can also be introduced into cultures from environmental sources, especially air, surfaces, or improper storage of specimens.

Yeast Contaminants

- Candida species:
  - Characteristics: Yeast-like fungi, gram-positive, non-filamentous.
  - Clinical Significance: Candida albicans is a common human pathogen, but other species, like Candida glabrata, may contaminate certain cultures (e.g., in urine or sputum samples).
  - Laboratory Diagnosis: Creamy white colonies on Sabouraud agar, germ tube test positive for C. albicans.

Mould Contaminants

- Aspergillus species:
  - Characteristics: Filamentous fungi with septate hyphae and conidia.
  - Clinical Significance: Aspergillus fumigatus is pathogenic, but other species, like Aspergillus niger, can be environmental contaminants.
  - Laboratory Diagnosis: Dark green or black colonies, conidial heads on microscopic examination.
- Penicillium species:
  - Characteristics: Filamentous fungi, typically blue-green colonies with a characteristic “brush” appearance of conidiophores.
  - Clinical Significance: Non-pathogenic but can contaminate fungal cultures.
  - Laboratory Diagnosis: Blue-green colonies, characteristic brush-like conidial heads.
- Cladosporium species:
  - Characteristics: Filamentous fungi with dark, branching conidiophores.
  - Clinical Significance: Usually a contaminant, though certain species can cause infections in immunocompromised individuals.
  - Laboratory Diagnosis: Black or olive-brown colonies, microscopically showing chains of conidia.
- Alternaria species:
  - Characteristics: Dark, septate hyphae with typical alternating conidia.
  - Clinical Significance: Opportunistic pathogen, but often a contaminant in laboratory cultures.
  - Laboratory Diagnosis: Dark, rough-textured colonies with a “feather-like” appearance.

Strategies to Minimize Contamination

To minimize contamination during microbiological work, the following practices are essential:

1. Aseptic Techniques:
  - Always use sterile equipment and media.
  - Practice proper hand hygiene and wear gloves when handling samples.
2. Proper Specimen Collection:
  - Use appropriate swabs, containers, and collection devices.
  - Avoid contamination from the skin or external sources by cleaning the collection site properly.
3. Environmental Controls:
  - Maintain a clean laboratory environment with proper ventilation.
  - Regularly disinfect surfaces and equipment.
  - Ensure that water used in cultures is sterile and free from contaminants.
4. Quality Control:
  - Regularly test for and monitor contamination in media and reagents.
  - Employ periodic checks of air quality and environmental sterilization procedures.

Distinguishing Contaminants from True Pathogens

- Clinical History: Carefully assessing the patient’s clinical history can help differentiate between contaminants (e.g., from normal flora) and true pathogens.
- Culture Growth Patterns: Some contaminants (like Staphylococcus epidermidis) show growth in cultures from non-infected sites. At the same time, true pathogens typically have distinctive growth characteristics or are obtained from sites of infection.
- Biochemical Testing: Detailed biochemical testing and molecular identification (e.g., PCR) can confirm whether a suspected pathogen is a contaminant.
- Repeat Cultures: If the same microorganism is isolated from multiple specimens, it is more likely to be a true pathogen. Contaminants are typically isolated from a single specimen.