The evolutionary consequences of environmental robustness in resistant bacteria have not been widely addressed. Bacteria able to robustly grow in a variety of antibiotic environments are generalists according to the ecological definition of niche breadth. This niche breadth may have been directly molded through selection if the bacteria descended from a lineage exposed to the various antibiotics. Alternatively, the niche breadth may be fortuitous if the bacteria obtained a plasmid harboring multiple-resistance genes that ran the gauntlet of selection in other contexts. No matter the circumstance, we noted above that some theory predicts evolved generalization may be particularly useful if organisms tend to encounter highly variable environments. 41 The phenomenon of drug cross-resistance and possible evolutionary modification of a resistance allele allowing survival in altered circumstances ͑i.e., evolutionary innovation͒ lend plausibility to the concept that multiple-resistance strains by an evolvability advantage would better thrive in the face of novel drugs. We are unaware of any studies addressing the relationship between number or diversity of resistance genes in bacteria and the evolvability of bacterial pathogens in new environments. Such research would be highly useful for examining the evolutionary processes underlying the widespread success of resistant bacteria.