Identification of Streptomyces spp. in a Clinical Sample: Always Contamination? Results of a French Retrospective Study

Abstract

Background: Streptomyces are environmental gram-positive bacilli that can cause ubiquitous mycetoma and, more rarely, invasive infections. We describe the clinical relevance of Streptomyces spp. identified in human samples and characteristics of patients with invasive Streptomyces infections.

Methods: We conducted a retrospective (2006-2017) study of Streptomyces isolates identified in clinical samples in French microbiology laboratories. Streptomyces genus was confirmed by a specific 16S rRNA polymerase chain reaction, and antibiotic susceptibility testing was performed by disk diffusion and trimethoprim-sulfamethoxazole minimum inhibitory concentration (E-test) if resistance was suspected. Patient characteristics, treatments, and outcomes were collected. Invasive infection was defined as a positive culture from a sterile site with signs of infection but without cutaneous inoculation.

Results: Of 137 Streptomyces isolates, all were susceptible to amikacin (113/113) and linezolid (112/112), and 92.9% to imipenem (105/113). Using disk diffusion, 50.9% (57/112) of isolates were susceptible to trimethoprim-sulfamethoxazole, but most of the apparently resistant isolates (25/36, 69.4%) tested by E-test were ultimately classified as susceptible. Clinical data were obtained for 63/137 (45.9%) isolates: 30 (47.6%) invasive infections, 8 (12.7%) primary cutaneous infections, 22 (34.9%) contaminations, 3 (4.7%) respiratory colonization. Patients with invasive infection were more frequently receiving corticosteroids than patients without invasive infection (11/30, 36.7%, vs 2/25, 8.0%; P = .03), and at 6-month follow-up, 14 of them were cured, 3 had relapsed, 4 were dead, and 9 were lost to follow-up.

Conclusions: Half of the clinical samples that grew Streptomyces were from patients with invasive infection. In that case, antimicrobial therapy should include 1 or 2 antibiotics among linezolid, amikacin, or imipenem.

Keywords: Actinobacteria; contamination; environment; invasive infection.

Figure 1.

Interpretation of trimethoprim-sulfamethoxazole susceptibility testing using a combination of the disk diffusion method and E-test strip on Mueller-Hinton agar plate read after 72 hours of culture. A, Comparison of a resistant isolate (left picture) and a susceptible isolate (right picture). B, Reading of the trimethoprim-sulfamethoxazole MIC using the 80% inhibition criterion. From left to right, trimethoprim-sulfamethoxazole MICs were (white arrows) 0.094, 0.25, and 0.125. Abbreviation: MIC, minimum inhibitory concentration.

Figure 2.

Susceptibility of 113 Streptomyces isolates to different antibiotics. A, Proportion of susceptible isolates (n = 113 or indicated between parentheses) using the disk diffusion on CA-MH agar plate method, read after 72 hours of culture. With this method, 50.8% (57/112) of the isolates were susceptible to trimethoprim-sulfamethoxazole. B, Results of the trimethoprim-sulfamethoxazole MIC measured by E-test strips on an agar plate. An isolate was considered susceptible if the trimethoprim-sulfamethoxazole MIC obtained was ≤2/38 mg/L (dotted line). Of note, among 55 isolates classified as being resistant using the disk diffusion method, only 36 resumed growth and could be tested using E-test strips. Isolates with trimethoprim-sulfamethoxazole MICs >32/608 are depicted as “32.” Abbreviation: MIC, minimum inhibitory concentration.