High genetic diversity of Staphylococcus aureus strains colonising the nasopharynx of Gambian villagers before widespread use of pneumococcal conjugate vaccines.

BACKGROUND: With the global efforts of reducing pneumococcal disease through widespread introduction of pneumococcal vaccines, concerns have emerged on the potential increase of morbidity and mortality from S. aureus disease. Little is known however, of the carriage rates of S. aureus or of its' relationship with carriage of S. pneumoniae in rural Africa, and West Africa in particular where very high rates of carriage of S. pneumoniae have been reported. This study aims to evaluate the prevalence, antibiotic susceptibility patterns and genotypes of S. aureus isolated from the nasopharynx of healthy individuals in rural Gambia before the introduction of routine use of pneumococcal conjugate vaccines in the country.

RESULTS: Overall prevalence of S. aureus nasopharyngeal carriage was 25.2%. All S. aureus isolates tested were susceptible to methicillin. Resistant was observed for sulphamethoxazole-trimethoprim (15%) and tetracycline (34.3%). We found 59 different sequence types (ST), 35 of which were novel. The most prevalent sequence types were ST 15 (28%) and ST 5 (4%). Eighty two percent (494/600) of study individuals were S. pneumoniae carriers with S. pneumoniae carriage rates decreasing with increasing age groups. S. aureus carriage among pneumococcal carriers was slightly lower than among non-pneumococcal carriers (24.3 versus 29.3%; p = 0.324). There were no associations of carriage between these two bacteria across the 4 age groups. However, analysis of pooled data children < 2 years and children 2 to < 5 years of age showed a statistically significant inverse association (24.1 and 50.0% for S. aureus carriage among S. pneumoniae carriers and non-carriers respectively; p = 0.015).

CONCLUSIONS: We report that nasopharyngeal carriage of S. aureus in rural Gambia is high in all age groups, with approximately 1 out of 4 individuals being carriers in the pre-pneumococcal vaccination era. There are indications that nasopharyngeal carriage of S.aureus could be inversely related to carriage of S. pneumoniae amongst younger children in The Gambian and that S. aureus clones in The Gambia show significant genetic diversity suggesting worldwide dissemination. Findings from this study provide a useful background for impact studies evaluating the introduction of pneumococcal vaccines or other interventions targeting the control of S. aureus infections and disease.