Surprisingly, the anti Sok PNAs caused a greater than ﬁve orders of magnitude reduction in viable cell counts at low micromolar concentrations. The high level of cell-killing observed indicates that the anti Sok PNAs are bactericidal and that the vast majority of cells retain the plasmid and capacity to produce Hok toxin during treatment. This suggests that effective competitive inhibitors of antitoxin RNAs can be designed and with further improvement may provide effective new antimicrobials. Recent studies using peritoneal infections in mice suggest that antisense agents based on PNA and morpholino chemistry are effective antimicrobials against E.coli when targeting essential genes (28,37). In the case of PNA and other types of antisense oligomers, further progress is needed to improve toxicity, dis- tribution and other drug properties to provide effective inhibitors for the clinic. We speculate that antitoxin RNAs may provide more accessible or sensitive targets relative to growth essential genes. Also, the emergence of drug resistance would seem unlikely as the target antitoxin struc- ture is constrained by a need to maintain recognition with the toxin encoding mRNA. Plasmid loss would provide an obvious resistance mechanism; however, the level of cell killing observed compares well with that of conventional antimicrobials, and targeting chromosome-encoded hok/sok or ldr loci would circumvent the problem of resistance by loss of the TA-carrying plasmid. Also we found that the binding-rate of PNA::Sok-RNA interaction is rapid, but slower than that for hok mRNA::Sok-RNA interactions. This result suggests that an excess of PNA relative to hok mRNA is needed to out-compete the native interactions in cells; however, a slow dissociation rate for PNA::Sok-RNA complexes and PNA accumulation is expected and this may also aid competition. Therefore, while we have observed hok activation in cells with the present inhibitor design, it may be possible to more efﬁciently trigger Hok-mediated cell death by using antisense agents with faster binding kinetics, although we suspect that cell delivery will remain the major limiting factor in this approach. Many TA loci are chromosomal in pathogenic E.coli (38). Previously, it was shown that the hok/sok TA gene stability element is widespread in E.coli plasmids (39) and that the E.coli genome encodes ﬁve hok/sok homologous loci (32). Using database searches we extend here these previous analyses to additional enteric bacteria and the results show a surprising abundance of homologous and analogous TA loci in pathogen genomes. Interestingly, while hok/sok loci in E.coli K-12 are inactive, those in the pathogenic Ochmann ECOR collection are active. However, it is important to consider that single copy chromosome-encoded toxin genes may be more difﬁcult to activate than plasmid copies. RNA::RNA interactions are inherently difﬁcult to study in cells. One general problem is that sequence mutagenesis alters both sequence and higher order folding, which could affect RNA expression, maturation and RNA half-life. Also, RNA interactions are transient and often involve RNA isoforms, as exempliﬁed by the hok/sok system. Therefore, additional methods to probe RNA interactions in living cells are needed. Competitive inhibition using antisense agents has been used to sequester microRNAs in mammalian cells (40), and the same basic approach should prove helpful in studies of RNA interactions in bacteria. 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