Overview
Conjunctivitis, commonly referred to as pink eye, is an inflammatory condition of the conjunctiva of the eye. It is caused by infectious agents such as bacteria, fungi, and other microorganisms and is known to be highly contagious. The normal ocular flora includes organisms such as Corynebacterium species (diphtheroids), coagulase-negative staphylococci, Neisseria species, Moraxella catarrhalis, and non-hemolytic streptococci.
A Conjunctival Culture and Sensitivity test is performed to isolate and identify pathogenic microorganisms responsible for infection and to guide appropriate antimicrobial therapy.
Symptoms
While the slides primarily focus on diagnostic aspects, conjunctivitis is characterized by inflammation of the eye, often presenting clinically with redness and irritation. Severe, refractory, neonatal, or atypical infections warrant microbiological evaluation using conjunctival culture, especially when infections do not respond to empirical treatment.
Causes
A wide range of bacterial and fungal pathogens may cause conjunctivitis.
Common bacterial causes include:
- Staphylococcus aureus
- Streptococcus pneumoniae
- Group A Streptococcus
- Neisseria gonorrhoeae
- Chlamydia trachomatis
- Klebsiella pneumoniae
- Pseudomonas aeruginosa
- Haemophilus influenzae
Fungal causes include:
- Sporothrix schenckii
- Blastomyces dermatitidis
- Paracoccidioides brasiliensis
- Microsporum canis
- Malassezia furfur
- Candida species
Risk Factors
Conjunctival culture is particularly indicated in:
- Severe or refractory conjunctivitis unresponsive to treatment
- Neonatal conjunctivitis to identify serious pathogens such as gonococci
- Immunocompromised patients who are at higher risk of atypical or resistant infections
- Pre- and post-ophthalmic surgical cases to prevent or detect surgical site infections
- Suspected fungal or unusual infections
- Outbreak or epidemic investigations to identify and control the spread of infection
Prevention and Diagnostic Approach
Proper specimen collection and laboratory handling are crucial for accurate diagnosis. Samples should be collected before initiating antimicrobial therapy, using strict aseptic technique. The lower eyelid is gently everted, and a sterile swab is rubbed over the conjunctival membrane while avoiding the cornea. The specimen is either placed in transport medium or directly inoculated onto culture media at the bedside.
Samples should be transported to the laboratory promptly. If delayed, refrigeration is preferred. Cultures are performed on blood agar, chocolate agar, MacConkey agar, Thayer-Martin agar, and Sabouraud Dextrose Agar (for fungal isolation), with appropriate incubation conditions.
Microorganism identification is carried out using:
- Conventional biochemical tests (e.g., TSI, urease, citrate, indole, catalase, and coagulase) require about 24 hours
- Automated methods such as VITEK 2 Compact (6–10 hours) and MALDI-TOF (approximately 2 hours)
Antimicrobial sensitivity testing is done using:
- Disc diffusion method, measuring zones of inhibition
- Automated systems, providing Minimum Inhibitory Concentration (MIC) values
If no growth is observed, a sterile report is issued by the third day. Bacterial culture reports with identification and sensitivity are available within seven days. Fungal cultures are followed for up to four weeks due to slow growth patterns.
Clinical Significance
Conjunctival culture plays a vital role in confirming infectious etiology, identifying causative organisms, guiding targeted antimicrobial therapy, diagnosing neonatal and severe infections, monitoring antibiotic resistance, and assisting in epidemiological surveillance.
