The finding of Pediococcus isolates that showed only moderate resistance to Vancomycin is discordant with other studies to date which have consistently reported the genus Pediococcus to be intrinsically Vancomycin-resistant [10, 12–14]. The isolates that were not resistant to all concentrations of Vancomycin tested were from the species P. acidilactici (N = 1), P. claussenii (Ropy, N = 1; Non-ropy, N = 3), P. damnosus (N = 1), and P. parvulus (Non-ropy, N = 2), suggesting that the phenomenon is not the product of a clonal event. It has previously been shown that intrinsic Vancomycin resistance in P. pentosaceus is due to a modified peptidoglycan precursor ending in D-Ala-D-lactate [15]. While this may also be the mechanism used by other Vancomycin-resistant pediococci, it is likely that the eight susceptible isolates do not possess this mechanism. Because media previously used for Pediococcus antimicrobial susceptibility testing have since been shown to be inappropriate for such testing (11), it is possible that the earlier finding of intrinsic Pediococcus Vancomycin-resistance was an artifact of the testing medium used, rather than reflective of pediococci genetic content.
The ropy phenotype did not associate with resistance to any of the antimicrobial compounds tested. This was an unexpected result as the ropy phenotype acts to create a biofilm which is expected to act as a physical barrier for the bacteria, putatively protecting them from the antimicrobial compounds. Why no associations were found is unclear. It may be that the type of exopolysaccharide matrix produced by these isolates did not result in a sufficiently dense matrix so as to inhibit the passage of antimicrobial compounds. Alternatively, the amount of energy expended on the production of exopolysaccharide may have caused a decreased ability to grow in the presence of the antimicrobial compounds, despite the partial antimicrobial barrier created by the exopolysaccharide.
Of particular interest to the brewing industry is the presence in pediococci of hop-resistance or beer-spoilage correlated genes (ABC2, bsrA, bsrB, hitA, horA, and horC). Of these six genes, only horA has been conclusively shown to function as a multidrug transporter, however, the ABC2, bsrA, and bsrB genes are highly similar to known ABC MDR genes, and the hitA gene is similar to divalent cation transporters. As such, all six of these beer-spoilage or hop-resistance correlated genes were assessed for associations with antimicrobial resistance. The genes hitA, horC, and ABC2 did not occur with sufficient frequency to determine statistical correlation [Additional file 2]. It is important to note that, as was found for ability to grow in beer, the bsrA, bsrB, and horA genes did not demonstrate significant associations with resistance to any of the antibiotics tested, but rather with susceptibility.
When MIC was compared to ability of isolates to grow in beer, eight of the 17 antibiotics that we tested surprisingly demonstrated a significantly lower MIC in isolates that could grow in beer. The eight antibiotics included Synercid, Ampicillin, Levofloxacin, Penicillin, Ciprofloxacin, Sulfamethoxazole/Trimethoprim, Gatifloxacin, and Oxacillin + 2% NaCl. This suggests that, despite repeated exposure to antimicrobial hop-compounds in the brewery setting, Pediococcus isolates capable of growing in the beer tend to be more susceptible to commonly used antimicrobial compounds than are isolates which cannot grow in beer. It is possible that this association may actually be independent of the presence of hop-compounds, instead being an indication of the environment encountered within the brewery environment by the beer-spoilage isolates. Although beer-spoilage bacteria must originate from outside the brewery, isolates capable of growing in beer have presumably become highly acclimatized or especially adapted to grow in the beer environment. Ideally, beer will not contain any wild yeasts or bacteria and, as such, contaminating pediococci are growing in an environment that does not contain a plethora of antimicrobial compounds naturally created by other organisms living in the same environment. Based on this scenario, Pediococcus isolates entering the brewery environment from outside sources (e.g., plant materials such as hop cones or barley) would possess mechanisms of resistance against multiple antimicrobial compounds. However, upon entering the brewery environment which should be free of other competing microbes, the pediococci would encounter no selective pressures other than hop-compounds and thus fail to maintain the genetic mechanisms for antimicrobial resistance.
It is curious to note that the bsrA and bsrB genes, hop-resistance, and beer-spoilage are all significantly negatively-associated with resistance to Ciprofloxacin. Moreover, although horA is strongly correlated to ability to grow in beer, this gene does not show any association (negative or otherwise) with Ciprofloxacin resistance. While the underlying mechanism for this association with lowered resistance to Ciprofloxacin is unknown, it strongly suggests that hop-resistance, and in turn beer-spoilage, is linked to the presence of the bsrA and bsrB genes, while the horA gene may simply be correlated by chance to ability of Pediococcus isolates to spoil beer. That is to say, because the bsrA and bsrB genes (like the beer-spoilage phenotype) are negatively correlated to ciprofloxacin resistance, while the horA gene is not, the bsrA and bsrB genes are likely more closely associated with beer-spoilage than is the horA gene.