The antibiotic-resistant strains S. aureusR and S. TyphimuriumR grew well in TSB at pH 5.5 compared to the antibiotic-susceptible strains (Table 3), suggesting that the antibiotic-resistant strains can adapt better to acidic conditions than the antibiotic-susceptible strains can. The acid-adapted cells provide cross-protection against heat, pH, osmolarity, and antibiotics (Leyer & Johnson, 1993; Lee et al., 1994; Greenacre & Brocklehurst, 2006). The biofilm formation by antibiotic-susceptible strains (S. aureusS and S. TyphimuriumS) was significantly inhibited by pH 5.5 compared to the antibiotic-resistant strains (S. aureusR and S. TyphimuriumR) (Table 3). The results imply that acidic pH can negatively influence biofilm formation (Salsali et al., 2006). However, acid-adapted antibiotic-resistant bacteria can be more resistant to other environmental stresses (Leyer & Johnson, 1993; Lee et al., 1994; Greenacre & Brocklehurst, 2006; McKinney et al., 2009). The MIC values of biofilm cells of S. aureus KACC13236 grown in TSB at pH 5.5 and 7.3 were relatively greater for all antibiotics than the values for planktonic cells (Table 4), indicating that biofilm cells were significantly more resistant to antibiotics compared with the planktonic cells. The results are in good agreement with previous reports that biofilm formation was directly associated with the significant increase in antibiotic resistance of bacteria (Donlan & Costerton, 2002; Kim & Wei, 2007; Cho et al., 2008; Kwon et al., 2008). The antibiotic resistance of biofilm cells might be attributed to their structural and physiological properties, leading to the changes in membrane permeability and metabolic activity (Costerton et al., 1999; Donlan & Costerton, 2002; Stewart, 2002). Compared to pH 7.3, the planktonic and biofilm cells grown in TSB at pH 5.5 were highly susceptible to the antibiotics used in this study (Table 5). Acid stress can cause the changes in cellular membrane permeability, leading to increased susceptibility to antibiotics (Alakomi et al., 2000; Delcour, 2009).
The norB and mdeA genes were stable in S. aureusS and S. aureusR planktonic cells cultured at pH 5.5 (Fig. 1a). The enhanced resistance to multiple antibiotics is mediated by the relative gene expression associated with norB, norC, and mdeA genes in S. aureus (Huang et al., 2004; Truong-Bolduc et al., 2006; Ding et al., 2008). The gene expression stability of norB, norC, and mdeA in S. aureus planktonic cells may play an important role in antibiotic resistance under anaerobic conditions, resulting in an increased virulence in S. aureus exposed to the gastrointestinal tract. Staphylococcal enterotoxins, a family of pyrogenic toxin superantigen-carrying staphylococcal pathogenicity island, are the major causative agents of staphylococcal food poisoning (Lowry, 1998; Becker et al., 2003; Derzelle et al., 2009). The relative expression levels of norB, norC, mdeA, sec, seg, sei, sel, sem, sen, and seo genes were increased 23.9-, 7.7-, 2.8-, 3.4-, 4.5-, 6.6-, 16.4-, 36.4-, 6.3-, and 8.2-fold, respectively, in the biofilm cells of S. aureusR grown in TSB at pH 7.3 (Fig. 1d). The efflux pump and virulence-related gene expression may be changed during the biofilm formation by S. aureusR. This confirms a previous report that the antibiotic resistance of biofilm cells contributed to the enhanced virulence (Rajesh & Vandana, 2009; Hoiby et al., 2010). The hilA and lpfE genes were overexpressed in S. TyphimuriumS and S. TyphimuriumR planktonic cells cultured in TSB at pH 5.5 (Fig. 2a). This suggests that the adhesion and invasion ability of S. Typhimurium can be enhanced under acid stress conditions (Chowdhury et al., 1996). The acrB and tolC genes were stable in S. TyphimuriumR grown in TSB at pH 5.5 (Fig. 2a). The AcrAB-TolC system is responsible for the increased antibiotic resistance, invasion ability, and virulence (Piddock, 2006; Nikaido et al., 2008; Pages & Amaral, 2009). Therefore, the observations imply that S. TyphimuriumR can effectively extrude antibiotics under acidic stress conditions. The AcrAB-TolC pump system can lead directly to multiple antibiotic resistance in bacteria (Piddock, 2006). Salmonella Typhimurium cells causing foodborne salmonellosis can invade the small intestine, which plays a role in bacterial pathogenicity (Pfeifer et al., 1999). The stn gene in S. Typhimurium is responsible for the production of enterotoxin (Chopra et al., 1994, 1999).
In conclusion, this study highlights the differential gene expression of the planktonic and biofilm cells of S. aureus (S. aureusS and S. aureusR) and S. Typhimurium (S. TyphimuriumS and S. TyphimuriumR) exposed to acidic stress under anaerobic conditions. The most significant findings in this study were that (1) the biofilm cells of multiple antibiotic-resistant S. aureusR and S. TyphimuriumR were more resistant to acidic stress compared with the planktonic cells; (2) the biofilm-forming ability was increased in S. aureusR and S. TyphimuriumR grown in TSB at pH 5.5 and 7.3; and (3) the relative expression of toxin-, virulence-, efflux pump-related genes in the biofilm of S. aureusR and S. TyphimuriumR strains was distinct from that in the planktonic cells. The multiple antibiotic-resistant pathogens (S. aureusR and S. TyphimuriumR) were more likely to form the biofilm, possibly leading to cross-protection against environmental stresses and enhanced pathogenesis. Further study is needed taking molecular approaches to elucidate the relationship between biofilm formation ability and the virulence potential of antibiotic-resistant foodborne pathogens exposed to various environmental stress conditions.