Over the past decade, the susceptibility of S. aureus to CHG and mupirocin has been decreasing, mainly owing to the prevalence of a variety of biocide resistance genes(24–26)]. It has been reported that these genes on plasmids or chromosomes are distributed worldwide and differ significantly throughout the different regions infected with S. aureus(27)]. From the data available, the plasmid-mediated qacA/B and smr genes prevailed at 35.4–41.6%, with 0–31% of S. aureus in the southern Asian region(27–29)]. In our study, only 10.11% and 1.39% of isolates carried qacA/B and smr genes, respectively, and 0.55% (n = 5) of isolates carried both genes simultaneously, which was lower than that reported by McNeil et al. for the Texas children’s hospital at 22.7% and 33.1% of isolates harboring qacA/B and smr genes, respectively, and 10.9% carrying both simultaneously(30)]. This could likely be owing to the lower prevalence of these genes in community-associated isolates compared to hospital-associated isolates. In addition, sporadic harboring of the smr gene is thought to confer less advantageous resistance compared to qacA/B(29)]. Plasmid-mediated genes, mupA and mupB, were detected in 2.35% and 0.42% of the isolates, respectively, and this prevalence was similar to that of clinical S. aureus isolates in children from Iran and France(31, 32)]. The qacG gene has been rarely reported, except in strains isolated from food sources and animals (33, 34)], and likewise, no qacG gene was found in any of the 722 S. aureus isolates analyzed in this study.
In parallel, a high distribution of chromosomally-encoded efflux genes, mepA (95.57%) and norA (78.81%), was determined in the present study, which was similar to those found in Europe, Africa, and Asia(15–17)]. However, compared to some European and African studies(15, 16)], a lower prevalence of lmrS (77.01%) and sepA (58.17%) was found in S. aureus isolates from children’s nasal swabs in Guangzhou, suggesting that the distribution of these genes varied by region. Overall, we provided updated data on the prevalence of biocide resistance genes among S. aureus colonies in the nasal cavity of school-aged children in Guangzhou, which showed large regional differences.
In addition, we found that the distribution of biocide resistance genes among S. aureus was related to the educational age and CC groups in this study. Although qacA/B or smr genes were reported to be more common in MRSA populations(35, 36)], our results showed that MRSA and MSSA populations have a similar distribution of these genes(P > 0.05).
To the best of our knowledge, the relationship between biocide resistance gene distribution among S. aureus and children’s educational age has scarcely been investigated. We found that among school-age students from elementary school through to senior school, junior school students had a significantly higher prevalence of qacA/B and mupA genes and senior school students had a higher prevalence of norA and sepA genes (P < 0.05), suggesting that there may be a wide distribution of biocide resistance genes in children of specific age groups.
MDR efflux pumps encoded by genes such as qacA/B, mepA, sepA, norA, lmrS, smr, qacG may lead to high-level resistance of S. aureus to multiple antibiotics(27)]. We found that qacA/B was related to resistance to erythromycin and clindamycin, which was consistent with the findings of Teresa et al.(15)]. However, unlike the known roles of mepA and norA in S. aureus to confer fluoroquinolone resistance (norfloxacin and ciprofloxacin)(27, 37)], the present study discovered that the mepA gene in S. aureus was positively associated with penicillin and clindamycin resistance, whereas the norA gene was inversely associated with resistance to penicillin, clindamycin, and erythromycin. However, these findings need to be further verified using a larger sample size. The smr gene has been reported to be related to ciprofloxacin resistance in other studies(30)], but it was not detected in our study, probably owing to its low prevalence.
Previously, Isa et al. revealed that higher MICs of chlorhexidine were exhibited among S. aureus isolates from CC5 and CC22 groups, which originated from humans and animals in Germany(38)], suggesting that the phenotype or genotype of biocide resistance may also be related to CC groups.
It is worth noting that among all CC genotypes of the 722 S. aureus isolates, the distribution of sepA was significantly low in CC30 (11.5%) and CC45 (10.1%) groups and high in CC5 (92.6%) and CC1 (98.8%) groups (P < 0.05), while the distribution of qacA/B (20.0%) and mupA (4.6%) genes had the highest proportion in the CC30 group. In addition, in this study we found that sepA was inversely associated with resistance to CLI and ERY, while qacA/B and mupA were positively associated with resistance to these antibiotics. These results explain our previous findings that CC30 genotypes had the highest proportion of ERY-and CLI-resistant S. aureus isolates, while CC1 possessed the lowest proportion(39)]. Moreover, we also revealed that the reason for the lower prevalence of sepA (58.17%) among S. aureus isolates found in Guangzhou compared to other regions (often as high as 90%)(15, 16)] was owing to varying distributions of this gene by region. The sepA gene was prevalent in CC5 (92.6%) and CC1 (98.8%) groups, but sporadically distributed in CC30 (11.5%) and CC45 (10.1%) groups (P < 0.05), while CC30 and CC45 were the main S. aureus CC genotypes carried by children in Guangzhou.
Considering qacA/B and mupA are well known to be highly associated with resistance to CHG and mupirocin(40, 41)], 85 isolates carrying qacA/B or mupA were selected to further test their phenotypes. The three major CC genotypes for CHG and mupirocin resistance were CC30, CC45, and CC5; this trend was consistent with the distribution of the whole population(39)]. In this study, 64 qacA/B genes were associated with CHG resistance in 94.1% (n = 68) of S. aureus isolates and the mupA gene was associated with resistance to mupirocin in 100% (n = 12) of the isolates (including 7 LLMR and 5 HLMR), which verified that the qacA/B and mupA genes were highly resistant to chlorhexidine and mupirocin. Notably, we found five S. aureus isolates harboring both qacA/B and mupA genes, four (80%) resistant to CHG, and five (100%) highly resistant to mupirocin, suggesting that the qacA/B gene may also increase high-level mupirocin resistance among mupA + S. aureus isolates. Furthermore, we discovered whether the co-occurrences of qacA/B and mupA may also promote resistance to multiple antibiotics; among the five qacA/B + mupA + isolates, three (60%) isolates were resistant to ERY and CLI, one (20%) isolate was resistant to OXA, SXT, and TCY, and two (40%) were intermediately resistant to RIF; this resistance rate was higher than when the two genes were present alone, but the underlying mechanism needs to be further explored.
Biofilms have been receiving much attention because of their association with resistance to antibiotic agents in S. aureus; strains within biofilms are thought to be hundreds of times more resistant to antibiotics than planktonic strains(19, 42)]. It was previously reported that a larger proportion (up to 90%) of S. aureus produced biofilms from nasal swabs compared to clinical isolates(43)]; in this study, we also found a high proportion of biofilm producers (98.8%) among qacA/B-positive and mupA-positive S. aureus isolates from nasal swabs of school-age children. Similar to the study by Barakat et al.(44)], we suggested that the biofilm formation ability of S. aureus was positively correlated with its resistance to CHG and mupirocin (P < 0.05). This finding raises additional concerns about the potential risks of S. aureus nasal colonization in school-aged children.
This study has several limitations. We only performed susceptibility to CHG and mupirocin and biofilm formation tests among S. aureus isolates carrying qacA/B and mupA. Thus, comparative analysis or a larger sample size will enhance the reliability of our results. Subsequently, we plan to further determine the distribution characteristics of S. aureus resistance to mupirocin and chlorhexidine in school-age children in Guangzhou and investigate the underlying mechanisms of mupirocin and CHG resistance in S. aureus.