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Very few data concerning the presence and the genetic lineages of S. aureus in wastewater are available in Tunisia. Therefore, this study was undertaken to expand the knowledge about S. aureus that might circulate among WWTPs. In this study, S. aureus was detected in 12 of the 62 wastewater samples collected (19.35%), and one isolate/sample was characterized: 11 isolates from La Charguia WWTP and one from El Menzah WWTP (Table 1). The percentage of samples carrying S. aureus detected in wastewater in this study is different from previous studies conducted in the United States and in Spain, where high rates (50–55%) of S. aureus occurrence in wastewater were observed (Rosenberg Goldstein et al. 2012; Gómez et al. 2016). MRSA was not detected in this study, in dissimilarity with some European studies where MRSA has often been reported in wastewater (Börjesson et al. 2010; Porrero et al. 2014). This finding might be related to the low rate or the absence of MRSA from healthy humans or animals in Tunisia (Ben Slama et al. 2011; Gharsa et al. 2015). Regarding antimicrobial resistance, all the isolates showed resistance to penicillin, which was encoded by the blaZ gene in seven isolates. Indeed, penicillin resistance is widely disseminated among S. aureus isolates worldwide (Lowy 2003). The msr(A) gene was detected in the seven erythromycin-resistant isolates (Table 1). The msr(A) gene codes for an ABC transporter protein that confers co-resistance to macrolides and streptogramin B antibiotics, and it has been reported from animal and human S. aureus isolates (Schmitz et al. 2000; Wendlandt et al. 2013, 2015). The lnu(A), lnu(B), vga(A) and vga(C) genes were not detected in the clindamycin-resistant isolate. Only one isolate was resistant to tetracycline and harboured the tet(K) gene. In fact, S. aureus resistance to tetracycline was more common in MRSA than in the MSSA isolates (Sharma et al. 2013). In general, tetracycline resistance in S. aureus is often encoded by the tet(M), tet(L) and tet(K) genes (Wendlandt et al. 2013, 2015). Only one isolate harboured the tst gene, and other virulence genes were not detected. Similarly, low rates of TSST-1 were earlier observed in S. aureus of wastewater origin (Gómez et al. 2016), in contrast to animal and human S. aureus isolates, where TSST-1 and other virulence factors are more frequent, especially in some specific genetic lineages (Monecke et al. 2009; Gharsa et al. 2012a).

Table 1

Characteristics of the 12 S. aureus isolates recovered in this study

StrainsWWTP samplesaspa-typeST/CCbagr-typePhenotype of resistancecResistance genesVirulence genes
C8300 C-AS t587 ST3245 NT PEN, ERY msr(A) – 
C8302 C-APT t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8303 C-AS t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8304 C-E t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8305 C-I t587 ST3245 agrII PEN, ERY msr(A) – 
C8306 C-APT t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8307 C-RS t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8308 C-I t674 ST15/CC5 agrII PEN, TET blaZ, tet(K) – 
C8309 C-APT t224 (CC97) agrPEN – – 
C8310 M-I t127 (CC1) agrIII PEN, CLIN blaZ – 
C8311 C-E t701 (CC6) agrPEN – – 
C8313 C-APT t1534 (CC522) agrPEN – tst 
StrainsWWTP samplesaspa-typeST/CCbagr-typePhenotype of resistancecResistance genesVirulence genes
C8300 C-AS t587 ST3245 NT PEN, ERY msr(A) – 
C8302 C-APT t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8303 C-AS t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8304 C-E t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8305 C-I t587 ST3245 agrII PEN, ERY msr(A) – 
C8306 C-APT t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8307 C-RS t587 ST3245 agrII PEN, ERY blaZ, msr(A) – 
C8308 C-I t674 ST15/CC5 agrII PEN, TET blaZ, tet(K) – 
C8309 C-APT t224 (CC97) agrPEN – – 
C8310 M-I t127 (CC1) agrIII PEN, CLIN blaZ – 
C8311 C-E t701 (CC6) agrPEN – – 
C8313 C-APT t1534 (CC522) agrPEN – tst 

aWWTP samples: the location of the WWTP (C: La Charguia; M: El Menzah) is indicated as well as the type of the samples (I, influent, APT, after primary treatment; AS, activated sludge; RS, returned sludge; E, effluent).

bNew ST is shown in bold. In some cases, the CC was assumed according to the spa-type (in this case it is shown in parentheses).

cPEN, penicillin; CLI, clindamycin; ERY, erythromycin; TET, tetracycline; NT, non-typeable.

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