Compared to previous MRSA clone studies in Taiwan, which showed only four major infection isolate clones (ST239, ST59, ST241 and ST5) [22, 23], our study revealed more heterogeneous MSSA lineages. The major PFGE cluster of MSSA infection isolates included ST1, ST6, ST7, ST12, ST188, and ST97. However, these STs have not been reported in MRSA isolates in Taiwan [22, 23], indicating that most MSSA lineages have a different genetic background compared to MRSA lineages. Additionally, these MSSA STs differ from the most prevalent S. aureus STs worldwide . Interestingly, the only ST common to MSSA and MRSA [22, 23] in Taiwan is ST59, the representative community-associated MRSA clone . Our findings are consistent with those from several other countries, suggesting that MRSA probably originated from MSSA clones through SCCmec transfer . These results, plus findings from a world-wide MRSA collection study, point to MRSA evolution in relatively few lineages , which allowed some successful clones to cause disease or spread. Although not all published studies have shown clonal differences between nasal carriage and infection isolates , our study revealed clonal differences in MSSA infection and nasal carriage isolates. MSSA infection isolates’ distinctive lineages and erythromycin susceptibility, as well as faster growth rates, support the clones’ uniqueness to cause community-onset MSSA infection.
Our finding that STs and clinical manifestations did not differ between community-associated and healthcare-associated community-onset isolates is consistent with Peacock et al.’s findings . That team found little difference between strains associated with disease in both community and hospital settings, and suggested that bacterial factors play a role in determining invasive disease. In the present study, PFGE cluster D: CC1 (ST188) was the predominant clone in the community-onset MSSA infections. Our previous study also showed that this clone predominated among MSSA isolates from inpatients in a 2002 collection from four hospitals located in the north, middle, south, and east regions of Taiwan . These results indicated that the epidemiology of MSSA infection isolates did not change much between these two periods, with no difference noted in either community or hospital settings.
Using PFGE or BURP analysis of spa types, we identified a few lineages likely to cause infection. Most of these representative CCs have been described previously as “group violations”, with the highest degree of misclassification between BURP groups and clonal lineages [12, 28]. However, these isolates may have clinical relevance because of their close relationship with severe community-associated infection strains in humans (MSSA476 and MW2 (USA400)) and ocular conjunctival infection in rabbits (UMCR1). All of these isolates belong to CC1 [29–31], and their high virulence has been shown in animal models [30, 32].
To clarify the difference between the nasal carriage isolates and infection isolates, we also compared the virulence gene profiles and growth rates among isolates. Infection isolates grew faster than carriage isolates. This difference suggests that infection isolates have better proliferation ability to outcompete other clones in the host. Additionally, the LukE-LukD leukotoxin (lukE-lukD) and gamma variant hemolysin (hlg-2) genes were more prevalent in infection isolates. Although seven virulence genes (egc enterotoxin genes, tst and hlg) were more prevalent in nasal carriage isolates, only isolates with egc enterotoxin genes were associated with slower growth rates (P < 0.01). It is possible that egc enterotoxin genes pose a fitness burden to the strains, resulting in slowed growth rates. The egc enterotoxin genes have been observed to have low-level production and weak virulence effects [33–35]. Our results reveal an epidemiologic relationship between egc enterotoxin genes and growth rate; however, the cause-effect relationship cannot be established in this study. It has been reported that virulence gene profiles are strongly linked with S. aureus clonal lineages . Our nasal carriage isolates have uncommon STs, with most possessing the egc enterotoxin genes. Previous studies also showed such an epidemiological phenomenon in nasal carriage isolates [5, 36]. However, the role of the egc enterotoxin genes in nasal carriage isolates is not yet understood. The reason why our nasal carriage isolates have higher erythromycin resistance rates compared to community-onset infection isolates is also unclear.
Although the PVL genes have been considered a marker for community-acquired MRSA , we did not find that PVL presence was associated with community-onset MSSA infections. Goering et al. also failed to find this PVL association in a recent study .
agr, a global virulence gene regulator of S. aureus, is strongly linked with clonal lineages and some disease syndromes . agr group I isolates were prevalent in our study, as in a previous study . A similar distribution of agr group I was observed among nasal carriage (65%) and infection isolates (74%). agr types and PVL toxin genes were not useful in predicting infection MSSA isolates in the community setting.
Though only 34 infection isolates were analyzed, these isolates originated from two medical centers over a four-month period. The low MSSA isolate numbers result from a high rate of methicillin resistance among S. aureus in Taiwan during the study period [6, 22]. Also, only community-onset MSSA isolates were collected for the present study.