Antone A. Medeiros

Failure of Macrolide Antibiotic Treatment in Patients with Bacteremia Due to Erythromycin-Resistant Streptococcus pneumoniae

The rate of macrolide resistance among Streptococcus pneumoniae is increasing, but some investigators have questioned its clinical relevance. We conducted a matched case-control study of patients with bacteremic pneumococcal infection at 4 hospitals to determine whether development of breakthrough bacteremia during macrolide treatment was related to macrolide susceptibility of the pneumococcal isolate. Case patients (n=86) were patients who had pneumococcal bacteremia and an isolate that was either resistant or intermediately resistant to erythromycin. Controls (n=141) were patients matched for age, sex, location, and year that bacteremia developed who had an erythromycin-susceptible pneumococcus isolated. Excluding patients with meningitis, 18 (24%) of 76 case patients and none of 136 matched controls were taking a macrolide when blood was obtained for culture (P=.00000012). Moreover, 5 (24%) of 21 case patients with the low-level-resistant M phenotype and none of 40 controls were taking a macrolide (P=.00157). These data show that development of breakthrough bacteremia during macrolide or azalide therapy is more likely to occur among patients infected with an erythromycin-resistant pneumococcus, and they also indicate that in vitro macrolide resistance resulting from both the efflux and methylase mechanisms is clinically relevant.

Novel plasmid-mediated beta-lactamase (MIR-1) conferring resistance to oxyimino- and alpha-methoxy beta-lactams in clinical isolates of Klebsiella pneumoniae.

Klebsiella pneumoniae isolates from 11 patients at the Miriam Hospital were identified as resistant to cefoxitin and ceftibuten as well as to aztreonam, cefotaxime, and ceftazidime. Resistance could be transferred by conjugation or transformation with plasmid DNA into Escherichia coli and was due to the production of a beta-lactamase with an isoelectric point of 8.4 named MIR-1. In E. coli, MIR-1 conferred resistance to aztreonam, cefotaxime, ceftazidime, ceftibuten, ceftriaxone, and such alpha-methoxy beta-lactams as cefmetazole, cefotetan, cefoxitin, and moxalactam. In vitro, MIR-1 hydrolyzed cephalothin and cephaloridine much more rapidly than it did penicillin G, ampicillin, or carbenicillin. Cefotaxime was hydrolyzed at 10% the rate of cephaloridine. Cefoxitin inactivation could only be detected by a microbiological test. The inhibition profile of MIR-1 was similar to that of chromosomally mediated class I beta-lactamases. Potassium clavulanate had little effect on cefoxitin or cefibuten resistance and was a poor inhibitor of MIR-1 activity. Cefoxitin or imipenem did not induce MIR-1. The gene determining MIR-1 was cloned on a 1.4-kb AccI-PstI fragment. Under stringent conditions, probes for TEM-1 and SHV-1 genes and the E. coli ampC gene failed to hybridize with the MIR-1 gene. However, a provisional sequence of 150 bp of the MIR-1 gene proved to be 90% identical to the sequence of ampC from Enterobacter cloacae but only 71% identical to that of E. coli, thus explaining the lack of hybridization to the E. coli ampC probe. Plasmid profiles of the 11 K. pneumoniae clinical isolates were not identical, but each contained a plasmid from 40 to 60 kb that hybridized with the cloned MIR-1 gene. Both transfer-proficient and transfer-deficient MIR-1 plasmids belonged to the N incompatibility group. Thus, the resistance of these K. pneumoniae strains was the result of plasmid acquisition of a class I beta-lactamase, a new resistance determinant that expands the kinds of beta-lactam resistance capable of spread by plasmid dissemination among clinical isolates. Images

The Risk of Endothelial Infection in Adults with Salmonella Bacteremia

Excerpt Arteritis and endocarditis are rare complications of salmonellosis. The diagnosis of such endothelial infections often requires invasive techniques. Frequently, the diagnosis is not establi...

Molecular Epidemiology of Antibiotic Resistance in Salmonella from Animals and Human Beings in the United States

We collected serotyped isolates of salmonella from reference laboratories in the United States, tested their susceptibility to antibiotics, and extracted plasmids from isolates that were resistant to a different combination of antibiotics from each of three serotypes. Restriction-endonuclease digestion showed that within each of the three groups, plasmid molecules from animal and human isolates were often identical or nearly identical. One serotype-plasmid combination appeared to be endemic in cattle in 20 states and infected 26 persons in two states. The human cases, which were not recognizably related except for their common plasmids, appeared to be clustered in time but geographically dispersed, like cases in previous outbreaks spread by food products. These findings suggest that resistance plasmids may be extensively shared between animal and human bacteria, and that spread of multiresistant strains of salmonella among animals and human beings, as observed in Britain, may have been undetected in the United States for lack of comparable surveillance.

SME-Type Carbapenem-Hydrolyzing Class A β-Lactamases from Geographically Diverse Serratia marcescens Strains

ABSTRACT Three sets of carbapenem-resistant Serratia marcescensisolates have been identified in the United States: 1 isolate in Minnesota in 1985 (before approval of carbapenems for clinical use), 5 isolates in Los Angeles (University of California at Los Angeles [UCLA]) in 1992, and 19 isolates in Boston from 1994 to 1999. All isolates tested produced two β-lactamases, an AmpC-type enzyme with pI values of 8.6 to 9.0 and one with a pI value of approximately 9.5. The enzyme with the higher pI in each strain hydrolyzed carbapenems and was not inhibited by EDTA, similar to the chromosomal class A SME-1 β-lactamase isolated from the 1982 London strain S. marcescens S6. The genes encoding the carbapenem-hydrolyzing enzymes were cloned in Escherichia coli and sequenced. The enzyme from the Minnesota isolate had an amino acid sequence identical to that of SME-1. The isolates from Boston and UCLA produced SME-2, an enzyme with a single amino acid change relative to SME-1, a substitution from valine to glutamine at position 207. Purified SME enzymes from the U.S. isolates had β-lactam hydrolysis profiles similar to that of the London SME-1 enzyme. Pulsed-field gel electrophoresis analysis revealed that the isolates showed some similarity but differed by at least three genetic events. In conclusion, a family of rare class A carbapenem-hydrolyzing β-lactamases first described in London has now been identified inS. marcescens isolates across the United States.

Emergence and nosocomial transmission of ampicillin-resistant enterococci.

Between 1986 and 1988, the incidence of ampicillin-resistant enterococci increased sevenfold at a university-affiliated hospital. Forty-three patients acquired nosocomial infections with ampicillin-resistant enterococci, most of which were also resistant to mezlocillin, piperacillin, and imipenem. An analysis of plasmid and chromosomal DNAs of isolates revealed that the increase was due to an epidemic of 19 nosocomial infections that yielded closely related strains of Enterococcus faecium and to a significant increase in the incidence of nonepidemic, largely unrelated strains of ampicillin-resistant enterococci. The nonepidemic strains were identified as E. faecium, E. raffinosus, E. durans, and E. gallinarum. A logistic regression analysis revealed that patients with nonepidemic resistant strains were 16 times more likely than controls to have received preceding therapy with imipenem. In our institution, the increase in the incidence of ampicillin-resistant enterococci appears to be due to the selection of various strains of resistant enterococci by the use of imipenem and to the nosocomial transmission of E. faecium and E. raffinosus. Images

Listeriosis in immunosuppressed patients: A cluster of eight cases

Bactermia due to listeria monocytogenes developed in eight patients who were receiving immunosuppresive medications during a 15 month period at one hospital. Seven survived. Meningitis was documented in only the four who received kidney transplants. Their neurologic signs were minimal, indicating a need to treat any immunosuppressed patient with Listeria bacteremia for meningitis. During this period the incidence of Listeria bactermia in immunosuppressed patients greatly exceeded that previously observed in this hospital or reported elsewhere, but the incidence of infection with other opportunistic agents was not increased. As with previously decreased listeria outbreaks in nonimmunosuppressed patients, no source or mechanism of spread could be identified. Thus, disease due to L. monocytogenes may occur focally among immunosuppressed populations, a pattern which also appears to be emerging for other opportunistic agents. A patients exposure to different opportunistic agents may be as important as the kind of immunosuppressive therapy he recieves in determining which opportunistic infection he will acquire or even whether any infection will occur.

Five novel plasmid-determined beta-lactamases.

Five novel plasmid-determined beta-lactamases named TLE-1, OXA-4, OXA-5, OXA-6, and OXA-7 were detected in ampicillin-resistant isolates of Escherichia coli and carbenicillin-resistant strains of Pseudomonas aeruginosa. TLE-1 resembled TEM-1 in substrate profile and reactions with inhibitors but differed in isoelectric point (5.55) and enzyme banding pattern on flat-bed electrofocusing.OXA-4, OXA-5, OXA-6, and OXA-7 hydrolyzed oxacillin, methicillin, and cloxacillin readily but differed from OXA-1, OXA-2, and OXA-3 in substrate profiles, inhibitor reactions, and isoelectric points (7.5 to 7.8).OXA-4 and OXA-6 were unusual for members of the OXA group in their sensitivity to inhibition by cloxacillin. OXA-5 and OXA-7 had isoelectric points close to that of SHV-1, emphasizing the need in beta-lactamase classification for studies in addition to isoelectric focusing. These five new enzymes bring the number of plasmid-determined beta-lactamases known in gram-negative organisms to more than 20. The evolution of such enzymatic diversity remains to be explored. Images

An Animal Source for the ROB-1 β-Lactamase of Haemophilus influenzae Type b

The most common cause of ampicillin resistance in Haemophilus influenzae type b is production of TEM-1 beta-lactamase; however, a novel enzyme with a similar substrate profile but a quite different isoelectric point has also been described. This beta-lactamase, designated ROB-1, has not been found previously in any other organism. In a survey of 46 ampicillin-resistant H. influenzae type b isolates, we found a second human isolate that produces ROB-1 and discovered that ampicillin-resistant isolates of the porcine pathogen Haemophilus pleuropneumoniae also produced ROB-1. In both Haemophilus species ROB-1 production was determined by plasmids that had considerable DNA sequence homology. However, the ROB-1 and TEM-1 beta-lactamase genes were not related. Our findings suggest that this form of ampicillin resistance has an animal reservoir and that conditions fostering its prevalence in animal strains may play a role in the spread of resistance to human pathogens. Images

Nosocomial Outbreaks of Multiresistant Bacteria: Extended-Spectrum Beta-Lactamases Have Arrived in North America

The history of the antibiotic era shows that widespread use of new antibiotics fosters emergence of resistant clinical isolates, often possessing novel antimicrobial resistance mechanisms. Clinical use of oxyimino--lactams, particularly ceftazidime and cefotaxime, skyrocketed during the last 15 years. In 1983 plasmid-mediated enzymes that confer resistance to oxyimino--lactamsextended-spectrum -lactamases (ESBL)were first observed in Germany. Since then, the number and variety of these -lactamases have increased rapidly and their distribution is now worldwide. Most of the ESBLs derive from TEM-1 or 2 and SHV-1, the common plasmid-determined -lactamases that confer resistance to penicillins and early-generation cephalosporins. Substitution of a single amino acid in the active site region of the SHV-1 molecule transforms it into the extended-spectrum -lactamase, SHV-2, that can inactivate late-generation cephalosporins. The number of derivatives of the TEM and SHV -lactamases proven to be unique by sequencing or oligotyping has now reached 26 and 5, respectively [1, 2]. Several have been found in many countries (SHV-2, SHV-4, SHV-5, TEM-6), whereas others seem to occur more commonly in one or two countries. For example, TEM-3 seems more prevalent in France and TEM-10 and TEM-12 in the United States and England [1]. Most of the isolates that produce ESBLs have come from hospitalized patients and have frequently caused nosocomial outbreaks, primarily due to Klebsiella pneumoniae strains that often have associated resistance to aminoglycosides. Recent surveys of nosocomial isolates of K. pneumoniae from Europe show that 14% to 16% produce ESBLs [3, 4]. In France the prevalence increased from less than 1% in 1985 to the current level by 1988. Extended-spectrum -lactamases are found much less frequently in other species of Enterobacteriaceae [3, 4]. General statistics regarding the prevalence of ESBLs, however, may be misleading. The prevalence of ESBLs among 12 French hospitals from different provinces varied from zero to 46% [4]. Indeed, three hospitals had a frequency much higher than the total prevalence of 14% and the others had much lower rates. In some hospitals, sporadic nosocomial outbreaks due to strains producing ESBLs seem to lead to an endemic problem. Selection pressure from widespread hospital use of late-generation cephalosporins apparently enhances colonization of the digestive or respiratory tracts of patients, and infection follows [2, 5]. Not surprisingly, outbreaks have often been associated with surgery, prolonged hospital stay, or the presence of urinary or arterial catheters, especially in patients in intensive care units [5]. Patients from nursing homes and other long-term care institutions may recirculate such strains into hospitals [6]. Most of the large outbreaks of nosocomial infection due to ESBL-producing Klebsiella have been reported from Europe, especially from France. In this issue, Meyer and colleagues [7] report the largest outbreak of ceftazidime-resistant Klebsiella infections yet to occur in a general hospital in North America. This report emphasizes that nosocomial infections caused by ESBL-producing bacteria are as serious a threat to hospitals in North America as they have been to hospitals in other countries. The outbreak in the Medical Center of Queens coincided with increasing use of ceftazidime and declined after restricting ceftazidime, implying a causal relationship. However, the simultaneous institution of barrier precautions with ceftazidime restriction undoubtedly contributed greatly to interrupting nosocomial spread. A casecontrol study might have provided more direct evidence of cause and effect. A recent outbreak in a pediatric hospital affords, perhaps, a clearer example of the danger of widespread use of ceftazidime [2]. Approximately 1 year after the hospital adopted a policy of using empiric ceftazidime monotherapy to treat febrile neutropenic patients, clusters of infections due to ceftazidime-resistant K. pneumoniae began to occur. The isolates produced a novel ESBL, TEM-26. After stopping the routine use of empiric ceftazidime monotherapy, the outbreak ended. Similarly, the widespread use of ceftazidime preceded an outbreak of ceftazidime-resistant K. pneumoniae in the Youville hospital, a long-term care and rehabilitation facility in Boston; the incidence declined after restricting ceftazidime use [8]. Interestingly, all three outbreaks involved Klebsiella strains that produced TEM-10 or TEM-26. Another important warning in the paper by Meyer and colleagues [7] is that the outbreak spread throughout the hospital for a year before it was detected. A major problem is the failure of clinical microbiology laboratories to detect strains producing ESBLs. Most of the ESBLs confer moderately high-level resistance to ceftazidime and aztreonam, but levels of resistance to cefotaxime may be marginal [1]. Laboratories that test only for cefotaxime will often fail to detect such strains. Measuring and recording inhibition-zone diameters would show the lower levels of resistance not detected by existing breakpoints. Using a test based on antibacterial synergy between -lactamase inhibitors and oxyimino--lactams, Sirot and colleagues [4] detected about 50% of the resistant strains misclassified as susceptible by the standard interpretative criteria of disc diffusion. As Meyer and colleagues point out, testing of susceptibility to ceftazidime is more likely to identify ESBL-producing strains. The same is true for aztreonam. Alternate methods of testing or new standards for interpreting existing tests are clearly needed to ensure early detection of multiresistant strains. Despite much accumulated experience in treating patients infected with ESBL-producing bacteria, few reports detail the specifics of therapeutic outcome with different -lactams [9, 10]. Consequently, little consensus exists on the efficacy of different -lactam antibiotics used alone or combined with -lactamase inhibitors against ESBL-producing bacteria. Many beta-lactams test susceptible with standard inocula but test resistant with larger inocula and show marginal efficacy in animal-model experiments [11, 12]. Using a more stringent criterion, the minimum bactericidal concentration in a macrodilution assay, Meyer and colleagues [7] found that only imipenem had consistent bactericidal activity. Although two patients with nonbacteremic bacteriuria responded to treatment with third-generation cephalosporins, imipenem gave the most favorable clinical results. More studies of the clinical efficacy of different antibiotic regimens in treating infections caused by ESBL-producing bacteria are needed. The cure by cefotaxime, after failure of ceftazidime, of a patient with meningitis due to an ESBL-producing Klebsiella (probably TEM-10) underscores the complexity of this issue [9]. The experience at the Medical Center of Queens illustrates the danger of blanket use of imipenem; namely, the subsequent emergence of an outbreak of infections caused by imipenem-resistant Acinetobacter. Antibiotic roulette is not the answer to limiting the threat of multiresistant microbes. Early detection of ESBL-producing strains and prompt containment through use of barrier precautions is essential to prevent widespread hospital outbreaks, along with judicious use of antimicrobial agents. Failure to control outbreaks has resulted in the appearance of novel ESBLs in the same institution. In Clermont-Ferrand Hospital, for example, eight different ESBLs appeared after the first surfaced in 1984 [13]. Both plasmid and strain dissemination occurred. The genes that encode at least two ESBLs reside on transposons, mobile genetic elements that can shuttle between plasmids and the chromosome [14, 15]. A TEM-1-bearing plasmid that had been seen 12 years earlier in one hospital recently underwent a single point mutation to produce TEM-12 [14]. Apparently, point mutations and recombination between different -lactamase genes are yielding new ESBLs in the hothouse environments where these genes are endemic [14, 16]. Chromosomal genes that encode even broader spectrum -lactamases have incorporated into plasmids, a long-feared threat. In The Miriam Hospital, nosocomial isolates of K. pneumoniae and Escherichia coli were found to produce the MIR-1 -lactamase, a plasmid-determined chromosomal-type -lactamase derived from Enterobacter cloacae [17]. The strains were highly resistant to all currently licensed -lactams except imipenem. Plasmid-borne genes determining other chromosomal-type -lactamases have also appeared recently in France, Greece, and Pakistan [18-20]. The discovery in Japan of a strain of Pseudomonas aeruginosa producing a plasmid-mediated -lactamase that confers resistance to imipenem and all other -lactams presents an even greater threat [21]. Early detection and prompt containment can limit the spread of these multiresistant pathogens.