Respiratory Culture, Rule Out Bacterial Pathogens

Structures adjacent to the respiratory tract, like the sinuses, are usually sterile or only transiently contaminated. They may become infected, often as a superinfection complicating upper respiratory viral infection. Cultures may be considered in patients who present with unusually severe signs and symptoms consistent with sinusitis, otitis media, or other pararespiratory infection, or when symptoms persist for more than 7 days.

The common bacterial pathogens are most commonly derived from the endogenous flora, including Moraxella catarrhalis, Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus. Anaerobic bacteria have been implicated, but usually with chronic infection or acute infection associated with trauma. Opportunistic molds, such as Mucor species, may cause severe, invasive upper respiratory tract infections in immunocompromised patients, especially in patients with DM.

Swab specimens should be considered unacceptable for culture, except for those collected by direct visualization by an otolaryngologist. Swab specimens are not optimal for isolation of anaerobic pathogens in chronic infections or acute abscesses.

Pus collected by surgical aspiration or drainage or by sinus aspiration should be transported to the laboratory under anaerobic transport conditions as quickly as possible.

Specimens are typically inoculated onto SBA and chocolate and MacConkey agar. Anaerobic media are inoculated if requested.

Turnaround time: Cultures are examined for at least 48 hours. Several days are required for isolation and identification, susceptibility testing, and further characterization of isolates.

Expected results: No growth, but light growth of endogenous respiratory flora is common.

Positive: Because common causes of pararespiratory infections are usually derived from upper respiratory tract endogenous flora, positive culture must be interpreted in the context of quantity or bacterial growth, purity of culture, Gram stain findings, and clinical signs and symptoms.

Common pitfall: Noninvasively collected specimens, which are more likely to represent endogenous rather than pathogenic flora, are often submitted for patient evaluation.