Characterization of Streptococcus pyogenes and Staphylococcus aureus associated with mixed skin and soft tissue infections in northern Saskatchewan communities

Abstract

Community-associated infections are a significant public health concern worldwide. In particular, epidemic and emerging antibiotic-resistant bacteria associated with skin and soft tissue infections (SSTI) are becoming more prevalent in certain populations. Bacterial pathogens Streptococcus pyogenes and Staphylococcus aureus can each cause infections that range from being mild in nature to severe and life-threatening. Together, these bacteria can be isolated from mixed, polymicrobial SSTI. These infections, such as impetigo, can be typically mild, but are easily spread by contact transmission. Treatment of these uncomplicated cases may include either prescription or over-the-counter (OTC) topical antibiotic therapy. In northern Saskatchewan communities experiencing high rates of SSTI caused by community-associated methicillin-resistant S. aureus (CA-MRSA), a high number of cases involving S. pyogenes and S. aureus mixed infections was identified. The objective of this work was to characterize S. pyogenes and S. aureus isolates associated with mixed SSTI by molecular typing and by testing antimicrobial susceptibility to systemic and topical antibiotics. Additionally, representative isolates were further characterized by whole genome sequencing and utilized in a murine skin infection model to investigate histopathologic and host response differences in mixed infections. During a 16-month period from 2005-2007, 118 cases of mixed SSTI cultured both S. pyogenes and S. aureus. Twenty-eight of these involved MRSA, while 90 cases isolated methicillin-susceptible S. aureus (MSSA) along with S. pyogenes. In S. pyogenes, resistance to bacitracin (0.7%), erythromycin (1.7%), clindamycin (1.7%), and fusidic acid (98.9%) was observed. All isolates were susceptible to retapamulin and antibiotic combinations contained in triple antibiotic OTC Neosporin® and Polysporin®. S. pyogenes emm typing and S. aureus spa typing revealed diverse populations of MSSA and S. pyogenes, but the combination of S. pyogenes emm type 41.2 and MSSA spa type t311 was the most frequently encountered (n=10). Genome sequence analysis of S. pyogenes strain 06BA18369 revealed an approximately 1.8 Mb genome that contains five putative prophages harbouring virulence factors including streptococcal superantigen-encoding genes. Additionally, this isolate harbours a clustered regularly interspaced palindromic repeats array suggesting ancestral bacteriophage exposure. The S. aureus strain 06BA18369 draft genome is approximately 2.8 Mb in size and contains four putative prophages. This isolate also harbours a staphylococcal cassette chromosome (SCC) with sequences homologous to SCC sequences of S. aureus, S. hominis, and S. epidermidis that includes a putative fusidic acid resistance gene. No transmission of genetic material was evident between the two species isolated from the same specimen. These two pathogens were able to develop lesions in the murine skin infection model; however, histopathologic and host immune response results were inconclusive. Both S. pyogenes and MSSA associated with mixed SSTI have diverse genetic backgrounds in contrast to the epidemic CA-MRSA. Genomic analysis of representative isolates revealed unique features in their mobile genetic elements. Topical OTC antibiotics may continue to be used for SSTI, but susceptibility patterns need to be continuously monitored for proper management of treatment. Future work is required to explore the genomic diversity of S. pyogenes and S. aureus strains circulating in the community.