The ∆ybeX strain is sensitive to ribosome-targeting
antibiotics
ybeX disruption has been reported to cause cell death in the
presence of chloramphenicol (Smith et al. , 2007). We therefore
explored the effects of various antibiotics on the ∆ybeX cells.
First, we determined the minimal inhibitory concentrations (MICs) in LB
for the WT and the ∆ybeX strains (see Materials and
Methods). The MICs were two times lower for ∆ybeX in the presence
of fusidic acid, clindamycin, chloramphenicol, tetracycline and
erythromycin (Table S1 ). These structurally unrelated
ribosome-targeting antibiotics have been shown to induce cold-shock
proteins or block the induction of heat-shock proteins (VanBogelen and
Neidhardt, 1990; Cruz-Loya et al. , 2019). We further inspected
the effects of these antibiotics using the dot spot assay described
above, except that the LB agar plates were supplemented with
sub-inhibitory concentrations of indicated antibiotics (see Materials
and Methods). The ∆ybeX strain exhibited severe sensitivity to
sub-lethal concentrations of all of these antibiotics (Fig.
3A ). Expressing the YbeX protein from a single-copy plasmid in the
absence of the inducer completely rescued the antibiotic sensitivity
(Fig. 3B ). In contrast, protein synthesis-targeting
antibiotics, for which we do not have evidence that they affect the cold
shock response (amikacin, streptomycin, kanamycin, tobramycin and
mupirocin), was founded not to have an effect. In addition, the
transcriptional inhibitor rifampicin revealed no effect on theΔybeX strain compared to the WT (Fig. 3B, Fig. S4b ). We
also measured the MICs in the MHB cation-adjusted media. The MICs for
wild type and ybeZ , and ybeX deletion strains remained the
same, while ybeY deletion strain showed lower MICs for all tested
antibiotics (data not shown).
We also tested the survival of two isogenic wild-type strains, MG1655
and BW25113 and the corresponding deletion strains∆ybeX::kanMG and∆ybeX::kanBW under sub-inhibitory antibiotic
concentrations. Both genetic backgrounds exhibited similar antibiotic
sensitivities, and removal of the kanamycin resistance cassette (in
strains ∆ybeX/-kanMG and∆ybeX/-kanBW ) had no effect (Fig. S4 ).
In contrast, ectopic expression of ybeX in the absence of an
inducer abolished the antibiotics sensitivity (Fig. 3B ).