Fig. 4

E. coli transition from rod to L-form and back on osmoprotective medium and in urine. Magenta arrows indicate individual bacteria, which were followed in time lapse and described in details in the main text. a Transition of strain ST782 from walled to L-form on osmoprotective medium (LM: L-form media) in the presence of 0.4 mg/ml phosphomycin. b Growth of strain ST782 in the presence of 0.4 mg/ml phosphomycin on non-osmoprotective medium (NA: nutrient agar). c ST782 transition from L-form to walled on osmoprotective medium (LM) after phosphomycin was removed. Time lapse over 5 h. Forty-minute increments are shown on the left-hand side of each image. d–g Survival of L-forms in urine and reversion to the walled state. d Phase contrast image of ST144 L-forms, which survived in urine overnight in the presence of phosphomycin. Viability of L-forms in urine was demonstrated by plating on NA (e) and LM (f). Growth was detectable only on osmoprotective medium (f) after 24 h incubation at 37 °C. g Microscopic appearance of bacteria detected in (f). h Model showing L-form switching as a mechanism for the recurrence of bacterial infections. Bacteria causing UTI are treated with cell-wall-targeting antibiotics. This leads to elimination of the cell wall and emergence of L-forms, which are not detectable by standard clinical culture methods. Following antibiotic treatment the bacteria can regenerate the wall and potentially cause recurrence of a full-blown infection. The bacterial cell wall is indicated with dark purple lines. Panel (h) was created by Katarzyna Mickiewicz. Scale bars = 5 µm