Cell growth of wall-free L-form bacteria is limited by oxidative damage

Abstract

The peptidoglycan (PG) cell wall is a defining feature of the bacterial lineage and an important target for antibiotics, such as β-lactams and glycopeptides. Nevertheless, many bacteria are capable of switching into a cell-wall-deficient state, called the "L-form" [1-3]. These variants have been classically identified as antibiotic-resistant forms in association with a wide range of infectious diseases [4]. L-forms become completely independent of the normally essential FtsZ cell division machinery [3, 5]. Instead, L-form proliferation is driven by a simple biophysical process based on an increased ratio of surface area to cell volume synthesis [6, 7]. We recently showed that only two genetic changes are needed for the L-form transition in Bacillus subtilis [7]. Class 1 mutations work to generate excess membrane synthesis [7]. Until now, the function of the class 2 mutations was unclear. We now show that these mutations work by counteracting an increase in the cellular levels of reactive oxygen species (ROS) originating from the electron transport pathway, which occurs in wall-deficient cells. Consistent with this, addition of a ROS scavenger or anaerobic culture conditions also worked to promote L-form growth without the class 2 mutations in both Gram-positive B. subtilis and Gram-negative Escherichia coli. Our results suggest that physiological compensation for the metabolic imbalance that occurs when cell wall synthesis is blocked is crucial for L-form proliferation in a wide range of bacteria and also provide new insights into the mode of action of antibiotics that target the bacterial cell wall.

Figure 1

Effects of ispA Mutation and Repression of PG Precursor Pathway in Protoplast Growth (A) Schematic representation of the requirements for cell proliferation in cell-wall-free B. subtilis. See text for details. (B) B. subtilis strains, BS115 (P_xyl-murE) and 4738 (P_xyl-murE ispA^∗ aprE::P_rpsD-mcherry), were grown in the walled state (with xylose), then converted to protoplasts, and incubated in nutrient broth (NB) and magnesium-sucrose-maleic acid (MSM) (no xylose) with PenG. After 5 hr, the cultures were mixed, and the cells were observed by time-lapse microscopy via a microfluidic system. Representative images are shown of overlays of the phase contrast (PC) and corresponding mCherry signals. Elapsed time (min) is shown in each panel. Lysed cells are labeled with arrowheads. The scale bar represents 5 μm. See also Movie S1.

Figure 2

Effects of ETC Activity on L-Form Growth (A) Schematic representation of the UPP and HPP synthetic pathways and ETC system in B. subtilis. Repression of various genes (indicated in red) supports L-form growth when combined with inhibition of the PG precursor pathway. See text for details. (B) Effects of repression of ispA, uppS, and hepS on cell growth in the absence of a cell wall. B. subtilis strains, wild-type (Wt; 168CA), YK1424 (P_spac-ispA), YK1889 (ΔuppS pLOSS-P_spac-uppS P_xyl-cdsA), and YK1450 (P_spac-hepS) were grown on nutrient agar (NA) and MSM plates containing 1% xylose with or without 1 mM IPTG and 400 μg/ml DCS (with 1 μg/ml 8j to prevent the rare reversion to walled cells) at 30°C. Note that the uppS lies immediately upstream of cdsA, which is essential for membrane phospholipid synthesis. To avoid the polar effect on the cdsA expression, we inserted a xylose-inducible promoter (P_xyl) in front of the cdsA gene, as described previously [10]. (C) PC micrographs of wild-type walled cells and L-forms (P_spac-ispA and P_spac-hepS) taken from the cultures shown in (B). The scale bar represents 5 μm. (D) Effects of transposon mutations on cell growth in the absence of a cell wall. B. subtilis strains, BS115 (Wt; P_xyl-murE), LR2 (ispA; P_xyl-murE ispA^∗), YK1816 (ndh; P_xyl-murE ndh::tn), YK1817 (qoxB; P_xyl-murE qoxB::tn), YK1818 (ctaB; P_xyl-murE ctaB::tn), and YK1522 (mhqR; P_xyl-murE mhqR::tn) were grown on NA and MSM plates with (MurE ON) and without (MurE OFF) 2% xylose at 30°C for 2 days (MurE ON) and 3 days (MurE OFF). (E) PC micrographs of L-forms taken from the cultures shown in (D). The scale bar represents 5 μm.

Figure 3

Increased ROS Production in Protoplasts and Its Suppression by an ispA Mutation (A) Expression patterns of several PerR regulated genes cultured in the walled (green; strain BS115; P_xyl-murE, 2% xylose), protoplast (yellow and red; strain BS115; P_xyl-murE, 2% or no xylose), or L-form (blue; strain LR2; P_xyl-murE ispA^∗, no xylose) states. See also Figure S1A and Table S1. (B) PC micrograph of a mixture of exponentially growing B. subtilis walled cells (YK2003; P_xyl-murE amyE::P_katA-gfp, 2% xylose) and an overnight culture of protoplasts (YK2003; no xylose) in NB and MSM (PC). The right panel shows the corresponding GFP image. The scale bar represents 5 μm. (C) Effect of ispA mutation on P_katA activity in protoplasts. PC micrograph of a mixture of overnight cultures of protoplasts (YK2003; P_xyl-murE amyE::P_katA-gfp, no xylose) and L-forms (YK2005; P_xyl-murE ispA^∗ amyE::P_katA-gfp aprE::P_rpsD-mcherry, no xylose) in NB and MSM (PC). The corresponding epifluorescence images of GFP and mCherry and the merge of GFP and mCherry are shown, respectively. The scale bar represents 5 μm. (D–F) The fluorescent probe C₁₁-BODIPY^581/591 was used as an indicator of lipid peroxidation. The probe undergoes a shift from red to green fluorescence emission upon peroxidation (see Supplemental Experimental Procedures). The scale bar represents 5 μm. (D) PC and corresponding epifluorescence micrographs (red and green fluorescence channels) of B. subtilis wild-type strain 168CA, grown in NB and MSM with (ii) and without (i) 1 mM H₂O₂. (E and F) PC and corresponding epifluorescence micrographs (red and green fluorescence channels) of overnight cultures of protoplasts (BS115; P_xyl-murE, E) or L-forms (LR2; P_xyl-murE ispA^∗, F) in NB and MSM without xylose. (G) The relative signal intensity of green fluorescence over total fluorescence (green + red fluorescence) of protoplasts (left; ispA⁺ [n = 201]) and L-forms (right; ispA⁻ [n = 199]). The signal intensity was obtained from similar images to (E) and (F) by ImageJ. Boxplots represent median (horizontal black lines), the upper and lower quartile values (boxes), and the most extreme data points within 1.5 times interquartile ranges (whiskers). The tendency of the weakness on the green fluorescent intensity in the L-forms is statistically significant (Student’s t test, p < 0.01). Student’s t test and the preparation of a boxplot were performed by the statistics software package, R.

Figure 4

Effects of ROS on L-Form Growth (A) Effect of repression of antioxidant systems on B. subtilis L-form growth in aerobic conditions. B. subtilis strains with the following mutations were cultured on NA and MSM plates with (MurE ON) or without (MurE OFF) 2% xylose at 30°C for 2 days in the presence or absence of 1 mM IPTG: P_xyl-murE ispA^∗ with P_spac-katA (YK2027), P_spac-sodA (YK2028), P_spac-bshB1/A (YK1604), and P_spac-zwf (YK1584). (B) E. coli L-form growth on L-form plates (NB and MSM 1% agar with 400 μg/ml fosfomycin) at 30°C for 3 days. Growth of the E. coli strain RM345 (ΔmurA containing the unstable plasmid pOU82-murA [3] on L-form plates with or without 5 mM reduced GSH in aerobic (i) or anaerobic (ii) conditions. See also Figure S2. (C) PC micrograph of E. coli L-forms taken from the cultures shown in (B). The scale bar represents 5 μm.