Colonies grown on TSBYE plates were screened for loss of chloramphenicol resistance and several sensitive clones were then examined by PCR to identify those in which an allelic exchange event had resulted in chromosomal

replacement of the wild-type copy of the gene with the see more mutant allele. This first round of allelic exchange mutagenesis led to the isolation of the derivative L. monocytogenes KD2812, which had a 627-bp deletion in the lmo2812 gene. The KD2812 single mutant was used in a second round of allele replacement mutagenesis, which began with the transformation of this strain with plasmid pADPBP5. Completion of the mutagenesis procedure led to the isolation of a double-mutant strain, L. monocytogenes AD07, which had a 627-bp deletion in the lmo2812 gene and a 1113-bp deletion in the lmo2754 (PBP5) gene. Characterization of KD2812 and AD07 selleck kinase inhibitor mutant strains To examine

the effect of PBP deletion on cell growth rate, the doubling times of cultures of EGD, KD2812 and AD07 were determined. The doubling time of the wild-type strain grown at 37°C was 40 min, whereas those of the single and double mutants were 45 and 50 min, respectively. These data indicate that the single and double PBP deletion strains grew significantly slower (P < 0.05) than EGD. The doubling time of the double mutant was also significantly different from that of KD2812. www.selleckchem.com/products/Belinostat.html Thus, although the bacteria were viable in the absence of Lmo2812 and PBP5, they grew more slowly than the wild-type. To determine the effect of these mutations on cell morphology, the strains EGD, KD2812 and DA07 were analyzed by scanning electron microscopy (SEM). As cells of the mutant strains displayed irregular morphology Racecadotril when grown at 42°C (Figure 3; h, i), the cell lengths were only determined when the strains were grown at 30 and 37°C. Cells of the L. monocytogenes strains lacking Lmo2812 were significantly longer than those of the wild-type (Student’s t test, P < 0.05) (Table 4). At 30°C the average cell length compared to strain EGD was increased by 38.5% in strain KD2812 and by 44.8% in the double mutant strain. The respective values at

37°C were 37.5% and 43%. The populations of the single and double mutant strains also showed some variation in cell morphology. A proportion of the cells of strain KD2812 showed an altered phenotype at each of the tested temperatures. The variant cells were characteristically curved with a bend at either one or both ends and subterminal constrictions. The number of cells with altered morphology was increased as the growth temperature was raised (Figure 3; b, e, h). Cell bending was more pronounced in the population of AD07 mutant cells (Figure 3; c, f, i). More than 90% of cells of the double mutant exhibited irregular morphology at 42°C. To determine whether disruption of the PBP-encoding genes had an impact on the β-lactam resistance of L. monocytogenes, microdilution MIC tests were performed.