Escherichia coli NCTC 86 organisms with impaired walls were prepared by three methods and the effect of antibacterial agents on their growth studied. The growth of penicillin spheroplasts was masked by the overgrowth of unaltered cells in the culture ; the EDTA-lysozyme spheroplasts themselves were non-viable. The growth of penicillin spheroplasts was not affected by cell-wall inhibitors and ampicillin suppressed the overgrowth of unaltered cells. The sensitivity of penicillin spheroplasts and parent cells to inhibition by a range of agents was similar. EDTA treatment enhanced the susceptibility of E. coli N C T C~~ and other strains of Gramnegative species to several antibiotics, particularly erythromycin. Polyacetic acid chelating agents related to EDTA and some new amides derived from glycine, alanine, phenylalanine or methionine also potentiated erythromycin in vitro. Erythromycin showed some activity in protecting mice against infection by EDTAtreated E. coli N C T C~~. The antibiotic did not protect against infection by the untreated bacteria and its activity was not greatly enhanced by simultaneous administration of EDTA or the amide derivatives.

Previous demonstrations of the irreversibility of dihydrostreptomycin transport across the cytoplasmic membrane of Escherichia coli were not due to decreases in the magnitude of the cytoplasmic membrane potential (delta psi). Irreversibility was probably not due to ATP hydrolysis being coupled to transport, because the rate of energy-dependent dihydrostreptomycin uptake was unaffected by 10-fold reduction in the cellular ATP level.

Twelve dissimilar clinical isolates and 4 type cultures of Pseudomonas aeruginosa have been repeatedly passaged on agar containing 200 pg carbenicillin/ml. Passaged variants were compared with control organisms for their sensitivities to a range of antibiotics initially by a multodisk test and subsequently by serial dilution in agar. Two of the variants, both derived from clinical isolates, showed pronounced increases in sensitivity to several antibiotics, particularly kanamycin, neomycin, gentamicin and colistin sulphate. In some instances the minimum inhibitory concentration (MIC) for the passaged variants was 32-64 times lower than that for the control organisms. These potentiations contrast with previous results obtained by other workers for P. aeruginosa. In addition, several other of our passaged variants developed a more moderate degree of enhanced sensitivity to a limited number of antibiotics. Eight (67%) of the clinical isolates and one type culture did not become more sensitive to any of the antibiotics tested following carbenicillin passage. Onset of increased antibiotic-sensitivity varied with the strain, particular antibiotic and medium employed for passage. Although the addition of sucrose (0.5 M) and magnesium sulphate (0.01 M) to the passage medium appeared to delay development of antibioticsensitivity their presence eventually encouraged larger potentiations in antibiotic activity. The significance of the conversion of P. aeruginosa into forms with increased susceptibility to several antibiotics during chemotherapy with carbenicillin is discussed.