In this study, we have demonstrated that the Type A F. tularensis tularensis strains are sensitive to Az in vitro. F. philomiragia and F. novicida are also sensitive with similar MICs. We determined that the MIC for F. tularensis LVS (NR-646) was 25 ug/ml Az, confirming the finding that LVS is relatively more resistant to Az than other Francisella strains.
Az is pumped out of gram-negative bacteria by several drug-efflux systems, including the RND efflux pumps. Az sensitivity differed VX-689 between F. novicida C59 wnt ic50 and F. tularensis Schu S4 RND efflux mutants. Wild-type F. tularensis Schu S4 has similar sensitivity to Az as wild-type F. novicida, but the RND efflux mutants ΔacrA and ΔacrB in F. tularensis Schu S4 are more sensitive to Az, whereas the F. novicida acrA and acrB mutants are more resistant. These F. tularensis Schu S4 ΔacrA and ΔacrB mutants were also Selleckchem BIBF-1120 reported to be more sensitive to the related antibiotic erythromycin . The difference between the F. tularensis Schu S4 and the F. novicida mutants might be due to the fact that F. tularensis Schu S4 has 254 pseudogenes; many of these genes are intact in F. novicida . For example, in F. tularensis Schu S4, at least 14 genes of the MFS transporter superfamily contain stop codons or frameshifts [34, 35] and are thus predicted to be
non-functional. Additional types of transporter proteins, including a drug-resistance transporter (FTT1618), are also reported to be non-functional pseudogenes  in F. tularensis Schu S4. It could be that the remaining TolC-AcrAB pump is the major means by which F. tularensis Schu S4 pumps out Az. If this pump is compromised, the organism would be more susceptible to the antibiotic, because it may not have an operational alternative pump, such as the MFS or ABC transporters to pump out the drug. This is supported by the finding that ΔacrA and ΔacrB mutants in F. tularensis Schu S4 also displayed increased sensitivity to nalidixic acid (a substrate for the MFS transporter), as well as detergents, streptomycin, tetracycline, and other molecules . In the case of F. novicida, there
may be alternate systems that can pump out the drug in the absence of the RND system. Alternatively, the mutation in acrA or acrB may cause an up-regulation of expression of another drug-efflux pump, rendering the bacteria more resistant to the antibiotic acetylcholine [36, 37]. Previous studies have shown that dsbB mutant in F. tularensis Schu S4 does not have any effect on antibiotic sensitivity (including the macrolide erythromycin) . Consistent with the F. tularensis Schu S4 dsbB mutant, the F. novicida dsbB mutant showed no difference from the wild-type F. novicida. Another common mechanism of resistance to macrolides is modification of the 23S rRNA. It has been reported that F. tularensis LVS has a point mutation in Domain V of the 23S rRNA, rendering it more resistant to erythromycin than F. novicida or F.