Know Thy Enemy: The Changing Susceptibility of Staphylococcus aureus

Staphylococcus aureus is a major pathogen in the adult and pediatric population. For several decades, there has been great concern over the rising prevalence of methicillin-

resistant S aureus (MRSA). This pathogen has been declining in adults, but it is not well documented whether the decrease in MRSA applies to children as well.

Dr Sutter and her colleagues evaluated the antimicrobial sus- ceptibility trends of S aureus isolates from infections in pediat- ric patients receiving care in the Military Health System treat- ment facilities from 2005 to 2014. Variations in antimicrobial susceptibility of S aureus between age groups, infection types, patient status (inpatient versus outpatient), and geographical region were examined.

In addition, the researchers evaluated the efficacy of non-β- lactam antibiotics in the treatment of MRSA and methicillin-sus- ceptible S aureus (MSSA) infections. Of note, isolates were only included if culture and site were consistent with S aureus infection so as to exclude colonization site or surveillance cultures.

Antibiotic susceptibility trends across the 10-year time period were analyzed by Cochrane-Armitage trend test. Fur- ther investigations included the analysis of covariates such as age and type of infection. Regional changes in oxacillin sus- ceptibility were identified through mean susceptibility rates, and a χ2 test was conducted to calculate P values for each of these comparisons.

From 2005 to 2014, there were 41,745 annual first positive S aureus isolates from 39,209 pediatric patients. Throughout the study period, 42% of isolates were MRSA, and 58% were MSSA. There was a significant trend of de- creased susceptibility to clindamycin, ciprofloxacin, and trimethoprim/sulfamethoxazole (TMP/SMX), although the overall rate of susceptibility of TMP-SMX remained quite high, with 98.4% of S aureus isolates susceptible in 2014. In addition, susceptibility to erythromycin, gentamicin, and oxacillin increased.

MSSA susceptibility to clindamycin declined from 90.7% to 83.8% (P < .0001), whereas this trend was not seen with MRSA. The majority of S aureus isolates were isolated from outpatients with skin and soft tissue infection, though skin

and soft tissue infection isolates were less likely to be oxacil- lin-susceptible than isolates from other infection types. Inpa- tient isolates were slightly more likely to be resistant to both oxacillin and clindamycin.

Interestingly, S aureus isolates from children to 5 years of age were much less likely to be susceptible to oxacillin in comparison to isolates from every other age group in the study. This decrease in oxacillin susceptibility may reflect higher rates of antibiotic usage in children 1 to 5 years of age, but it might also be due to the epidemiology associated with this group. Furthermore, toddlers have a high incidence of MRSA buttock/perineal abscesses, which might account for the differences.

Limitations to this study reflect the nature of a retrospec- tive review of laboratory data with limited clinical data. Also, susceptibility data did not differentiate between inducible and constitutive clindamycin resistance.

These data reflect the recent reports of declining rates of MRSA in the pediatric population and are similar to data re- ported in the adult literature. It is worth remembering that children 1 to 5 years of age had the highest rates of MRSA. This paper also notes the progressive decrease in clindamycin susceptibility in MSSA, with less than 84% of isolates report- ed as clindamycin-susceptible by 2014. Although it seems pre- mature to abandon this effective antibiotic within this setting, it is prudent to be aware of local susceptibility rates and select therapies accordingly. n

Jessica Tomaszewski, MD, is an Assistant Clinical Pro- fessor of Pediatrics at the Sidney Kimmel Medical College at Thomas Jefferson University in Philadelphia, Pennsylvania, and a hospitalist pediatrician at Nemours/Alfred I. duPont Hos- pital for Children in Wilmington, Delaware.

Charles A. Pohl, MD—Series Editor, is a Professor of Pe- diatrics, Senior Associate Dean of Student Affairs and Career Counseling, and Associate Provost for Student Affairs at the Sid- ney Kimmel Medical College at Thomas Jefferson University in Philadelphia, Pennsylvania.