In 1996, Serratia marcescens KU3838 was isolated from the urine of a patient with a urinary tract infection at a hospital in northern Japan and was found to contain the plasmid pKU501. Previously, we determined that pKU501 carries bla IMP and the genes for TEM-1-type ␤-lactamases as well as producing both types of ␤-lactamases (H. Yano, A. Kuga, K. Irinoda, R. Okamoto, T. Kobayashi, and M. Inoue, J. Antibiot. 52:1135-1139, 1999). pKU502 is a recombinant plasmid that contains a 1.5-kb DNA fragment, including the metallo-␤-lactamase gene, and is obtained by PCR amplification of pKU501. The sequence of the metallo-␤-lactamase gene in pKU502 was determined and revealed that this metallo-␤-lactamase gene differed from the gene encoding IMP-1 by one point mutation, leading to one amino acid substitution: 640-A in the base sequence of the IMP-1 gene was replaced by G, and Ser-196 was replaced by Gly in the mature enzyme. This enzyme was designated IMP-6. The strains that produced IMP-6 were resistant to carbapenems. The MICs of panipenem and especially meropenem were higher than the MIC of imipenem for these strains. The k cat /K m value of IMP-6 was about sevenfold higher against meropenem than against imipenem, although the MIC of meropenem for KU1917, which produced IMP-1, was lower than that of imipenem, and the MIC of panipenem was equal to that of imipenem. These results support the hypothesis that IMP-6 has extended substrate profiles against carbapenems. However, the activity of IMP-6 was very low against penicillin G and piperacillin. These results suggest that IMP-6 acquired high activity against carbapenems, especially meropenem, via the point mutation but in the process lost activity against penicillins. Although IMP-6 has reduced activity against penicillins due to this point mutation, pKU501 confers resistance to a variety of antimicrobial agents because it also produces TEM-1-type enzyme.Carbapenems such as imipenem, panipenem, and meropenem are the most potent agents for treatment of gramnegative infections due to the stability of these agents against the majority of ␤-lactamases and their high rate of permeation through bacterial outer membranes. In addition, imipenem, panipenem, and meropenem have a high affinity for penicillinbinding protein (PBP) 2 of gram-negative bacteria except for Pseudomonas aeruginosa. In P. aeruginosa, imipenem and panipenem have a high affinity for PBP 2 and meropenem has an affinity for PBP 3 (6, 19, 40).However, some clinical isolates have been reported to be resistant to carbapenems due to production of metallo-␤-lactamases. There have been increasing reports, especially from Japan (7,14,21,27,35), of gram-negative bacteria that carry the transferable carbapenem resistance gene bla IMP , including isolates of P. aeruginosa and Serratia marcescens. The metallo-␤-lactamases are class B enzymes in Ambler's molecular classification (1) and belong to Bush group 3 (3,31). Most of these metallo-␤-lactamases confer resistance not only to carbapenems but also to other ␤-lactams an...

A multiple-primer PCR was used to identify genes encoding aminoglycoside-modifying enzymes in 381 clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA). The technique used three sets of primers delineating specific DNA fragments of the aph(3)-III, ant(4)-I, and aac(6)-aph(2؆) genes, which influence the MICs of gentamicin, tobramycin, and lividomycin. Isolates with none of the three genes detected were susceptible to all three agents. Isolates with the aph(3)-III gene showed resistance to lividomycin (MIC > 1,024 g/ml), and those with the ant(4)-I gene were resistant to tobramycin (MIC > 8 g/ml). Isolates with only the aac(6)-aph(2؆) gene were resistant to gentamicin (MIC > 8 g/ml) and tobramycin in decreasing order; those with both the ant(4)-I and aac(6)-aph(2؆) genes also were resistant to gentamicin and tobramycin, but in increasing order. Susceptibility testing, then, could detect specific genes. In 381 Japanese MRSA isolates, the ant (4)

The ampC and ampR genes ofEnterobacter cloacae GN7471 were cloned into pMW218 to yield pKU403. Four mutant plasmids derived from pKU403 (pKU404, pKU405, pKU406, and pKU407) were isolated in an AmpD mutant ofEscherichia coli ML4953 by selection with ceftazidime or aztreonam. The β-lactamase activities expressed by pKU404, pKU405, pKU406, and pKU407 were about 450, 75, 160, and 160 times higher, respectively, than that expressed by the original plasmid, pKU403. These mutant plasmids all carried point mutations in theampR gene. In pKU404 and pKU405, Asp-135 was changed to Asn and Val, respectively. In both pKU406 and pKU407, Arg-86 was changed to Cys. The ease of selection of AmpR mutations at a frequency of about 10−6 in this study strongly suggests that derepressed strains, such as AmpD or AmpR mutants, could frequently emerge in the clinical setting.

This study describes the use of the polymerase chain reaction (PCR) to detect the tet(K) and tet(M) tetracycline resistance genes in Staphylococcus aureus. Primers based on the DNAsequence of the tet(K) and tet(M) genes from S. aureus were used as primers in the PCRassay to detect the presence of genes for resistance to tetracycline and minocycline. Two-hundred and fifteen isolates of S. aureus from Asian countries as Japan, Indonesia, China, Korea and Thailand were examined, and the results confirm that tet(K) specifies resistance to tetracycline but not to minocycline and tet(M) specifies resistance to both tetracycline and minocycline. Weobserved two different types of clinical isolates of S. aureus strains resistant to minocycline and tetracycline: the first carried only the tet(M) gene, while the second carried both the tet(M) and the tet(K) genes. Almost all of the clinical isolates of S. aureus resistant to minocycline and tetracycline from Indonesia, China and Thailand carried both tet(M) and tet(K) genes, while most of clinical isolates of S. aureuss from Japan and Korea carried only tet(M) gene.

To investigate how bacterial pathogens spread from child to child in a day care center, we monitored six children, two boys and four girls, born between August 1995 and November 1997, attending a day care center and analyzed nasopharyngeal samples from them using pulsed-field gel electrophoresis (PFGE). We obtained nasopharyngeal cultures from all of the affected children and almost all of the unaffected children between September 1998 and March 1999 after some children presented simultaneously with purulent rhinorrhea. Moreover, when a child was found to have acute otitis media, nasopharyngeal secretions from the child were independently cultured during treatment. During this period, 28 isolates of Moraxella catarrhalis, 13 ofStreptococcus pneumoniae, and 4 of Haemophilus influenzae were recovered. PFGE gave 8 patterns for M. catarrhalis, 10 for S. pneumoniae, and 1 for H. influenzae. PFGE patterns demonstrated spread of M. catarrhalis between children. However, each occurrence of clusters of infection with M. catarrhalis lasted 2 to 6 weeks, with a change in PFGE pattern between occurrences of clusters. The M. catarrhalis strain infecting each child also changed. Similarly, the S. pneumoniae strain in each child also changed. In contrast, infection with H. influenzaepersisted for about 3 months in an affected child.