Antibiotic resistance and carriage patterns of staphylococcus aureus among community-dwelling older adults in Hong Kong

Abstract

Staphylococcus aureus (S. aureus) is both a commensal bacterium and an opportunistic pathogen. Nasal carriage of S. aureus is a well-documented risk for subsequent infections. Longitudinal studies on S. aureus carriage revealed three patterns of carrier states, including persistent, intermittent and non-carriers. These different carrier states mean that the risk of S. aureus infection is different in each colonized individual. Persistent carriers were at greater risk of infections. However, there is no unified definition for a persistent carriage state. Hence, I conducted a systematic review and meta-analysis on the definitions and prevalence of persistent S. aureus carriage among healthy individuals. I found four important components, i.e. the length of follow-up period, sampling intervals, total number of swabs collected and the proportion of swab cultures being positive (carrier index), in the constitution of the definition of persistent carriers in the review of 82 eligible studies. High heterogeneities were observed in the pooled estimates for three subsets of studies with the same follow-up length. The pooled prevalence for persistent S. aureus carriers with one month or one year of follow-up was 21.9% and 19.7% respectively, and for persistent methicillin-resistant S. aureus (MRSA) carriers with one year of follow-up was 11%. Elderly people represent a particularly high-risk group that they not only have a higher carriage rate of S. aureus to spread to others but also contribute to a larger proportion of patients suffering from life-threatening complications than other age groups. However, there is a lack of information regarding S. aureus carriage among elderly community dwellers in Hong Kong. I then carried out a cross-sectional study to determine the prevalence of S. aureus nasal carriage, their antibiotic-resistance patterns and risk factors among healthy older adults aged over 60 years, recruited from elderly community centres. The prevalence of S. aureus and multidrug-resistant S. aureus (MDRSA) were 15.6% and 3.4% respectively among the 205 elderly participants. Different resistance patterns were observed in the MDRSA isolates but all showed resistance to macrolides. Being a current drinker (OR =3.17, 95% CI: 1.02-9.82) was the only risk factor found to be associated with S. aureus colonization. I further followed up on the identified S. aureus elderly carriers for two additional samplings over 8 months to explore their long-term carriage pattens. I used the Kaplan-Meier model to estimate the proportion still carrying S. aureus among twenty-five carriers who completed at least one follow-up and the median duration for S. aureus carriage was around 7 months. In addition, three elderly participants showed change in the antibiotic-resistance profiles of their S. aureus isolates. Given the changing phenotypes of S. aureus among the long-term elderly carriers in this study, further investigation in the population genetic structure (e.g. strain types, resistant genes and virulent factors) of S. aureus is needed to characterize variation of the resident strains. Understanding the dynamics of S. aureus carriage among healthy older adults will yield valuable information on devising appropriate strategies to curb the transmission of drug-resistant strains in the local community.