Serum dilution test for bactericidal activity. II. Standardization and correlation with antimicrobial assays and susceptibility tests.
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Anti-Bacterial Agents↗
Biomedical subjects
C W Stratton
Publications and source records attributed to C W Stratton.
Comparison of the bactericidal activity of cefotaxime and desacetylcefotaxime alone and in combination against Bacteroides fragilis group organisms.
Through the use of time-kill kinetic studies, the bactericidal activity of cefotaxime (CTX) and desacetylcefotaxime (dCTX) alone and in combination against 18 strains of Bacteroides fragilis group was studied. Each isolate was tested at subinhibitory, inhibitory, and suprainhibitory concentrations of each drug as determined from the MIC values. Overall CTX was more bactericidal than dCTX at each of the three concentration levels tested. The combination of CTX and dCTX showed comparable bactericidal activity to CTX at the subinhibitory and inhibitory concentrations, even though each component was present at only one-half the concentration of CTX alone. At suprainhibitory concentrations, the combination of CTX/dCTX appeared synergistic since the combination with each component at a concentration of 1 x MIC was as bactericidal as CTX at a concentration of 4 x MIC. CTX and dCTX alone and in combination exhibited comparable bactericidal activity against test isolates with high (greater than or equal to 32 micrograms/ml) or low (less than or equal to 16 micrograms/ml) MICs. Thus, in vitro the combination of CTX and its naturally occurring metabolite dCTX interacts to produce an additive or synergistic effect against strains of B. fragilis group. Whether the in vitro testing of the combinations is more relevant to clinical outcome than testing CTX alone needs further study.
Bacteroides↗
Evaluation of cefotaxime alone and in combination with desacetylcefotaxime against strains of Staphylococcus aureus that produce variants of staphylococcal beta-lactamase.
We evaluated cefotaxime (CTX) alone and in combination with its metabolite, desacetylcefotaxime (dCTX) against strains of Staphylococcus aureus that produce the four recognized variants of staphylococcal beta-lactamase and a beta-lactamase-producing isolate characterized by the expression of borderline resistance to methicillin. Although macrodilution MICs revealed that dCTX was less active than CTX against these strains (geometric means of 16 micrograms/ml and 4 micrograms/ml, respectively), the addition of clinically achievable concentrations of dCTX to CTX resulted in a reduction in the observed CTX MICs. This effect was similar to although less pronounced than that obtained by combining clavulanic acid with cefazolin. The increased antistaphylococcal activity noted by MIC determinations was confirmed with kill-kinetic studies. Determination of the relative rates of hydrolysis of selected cephalosporins showed that neither CTX nor dCTX were appreciably hydrolyzed by the variant staphylococcal enzymes. Evaluation of the effect of CTX and dCTX upon the staphylococcal beta-lactamases demonstrated that neither agent inhibited the destruction of a 100 microM solution of nitrocefin, although the reduction of CTX and cefazolin MICs by low concentrations of dCTX suggests that the dCTX metabolite may act as a competitive inhibitor of beta-lactamase. These observations may explain the previously demonstrated clinical efficacy of CTX used alone for the treatment of serious infections caused by S. aureus.
Cefazolin↗
Bactericidal activity of lomefloxacin SC 47111 (NY-198) and ciprofloxacin against selected pathogens.
The bacterial activity of lomefloxacin and ciprofloxacin against selected pathogens was compared using kill-kinetic methods to assess inhibitory (1 x MIC) and suprainhibitory (4 x MIC) concentrations. Five strains each of the following microorganisms were studied: Streptococcus pneumoniae, Streptococcus pyogenes, Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. We found that lomefloxacin was 8- to 16-fold less active than ciprofloxacin as measured by MICs. However, the bactericidal activities of both lomefloxacin and ciprofloxacin were comparable when concentrations 1 x MIC and 4 x MIC were tested. For many of the isolates tested, such concentrations would be achieved clinically. The low MICs seen for the Gram-negative bacilli did not correlate with killing ability for either drug: concentrations 8-16 x MIC were needed for 99.9% killing of the final inoculum. Although higher concentrations of lomefloxacin are needed for inhibitory and bactericidal activity, the improved pharmacokinetics of lomefloxacin may result in this agent being comparable to ciprofloxacin.
Anti-Infective Agents↗
Differences in the in vitro inhibitory and bactericidal activity of ceftizoxime, cefoxitin, cefotetan, and penicillin G against Bacteroides fragilis group isolates. Comparison of time-kill kinetic studies with MIC values.
Nineteen strains of the Bacteroides fragilis group were used to determine the bactericidal activity of ceftizoxime, cefoxitin, cefotetan, and penicillin G with time-kill kinetics studies. Each antimicrobial agent was tested at subinhibitory (1/2 X MIC), inhibitory (1 X MIC), and suprainhibitory (4 X MIC) concentrations. Penicillin G exhibited virtually no sustained bactericidal activity at any of the antimicrobial concentrations tested. At subinhibitory concentrations, ceftizoxime was considerably more bactericidal than cefoxitin or cefotetan: At 12 hr, ceftizoxime killed 89% of the inoculum, whereas cefoxitin and cefotetan killed 35% and 33% of the inoculum, respectively. At inhibitory concentrations, ceftizoxime was again more bactericidal than cefoxitin and cefotetan: At 12 hr, ceftizoxime killed 90% of the inoculum, whereas cefoxitin and cefotetan killed 78% and 73%, respectively. At suprainhibitory concentrations, all three antimicrobial agents showed comparable bactericidal activity at 12 and 24 hr. Ceftizoxime and cefoxitin had somewhat lower killing rates overall against test strains with high MICs (greater than or equal to 32) versus low MICs (less than or equal to 16). However, at subinhibitory concentrations, ceftizoxime killed the B. fragilis group strains with high or low MIC values more effectively than cefotetan killed strains with low MICs. At the highest antibiotic concentrations tested (4 X MIC), only slight differences were seen in the bactericidal activity of the three compounds, regardless of MICs.
Bacteroides↗
Effect of human serum on the bactericidal activity of daptomycin and vancomycin against staphylococcal and enterococcal isolates as determined by time-kill kinetic studies.
The bactericidal activity of daptomycin and vancomycin alone in cation-supplemented Mueller-Hinton broth and in human serum against clinical isolates of Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecalis was evaluated by exposing replicating microorganisms to concentrations ranging from 2 to 128 micrograms/ml for 24 hr. In addition, the possibility of emergence of resistance, the stability of each agent in the respective medium, and the percent of protein binding by human serum for each agent was evaluated. We found that a concentration of less than or equal to 8 micrograms/ml of daptomycin was sufficient to achieve bactericidal activity (greater than or equal to 99.9% killing of the inoculum) in cation-supplemented Mueller-Hinton broth for all staphylococcal isolates tested; a concentration of less than or equal to 16 micrograms/ml of daptomycin was required for bactericidal activity in cation-supplemented Mueller-Hinton broth for enterococcal isolates. In human serum, comparable bactericidal activity with daptomycin was achieved only with concentrations 8-16 times higher. A similar but less pronounced effect in human serum was seen for vancomycin. Neither daptomycin nor vancomycin was appreciably degraded in human serum over a 24-hr period. It is likely that the clinical efficacy of daptomycin in humans would be enhanced by higher dosing than has been studied to date.
Anti-Bacterial Agents↗
Susceptibility testing of Listeria monocytogenes. A reassessment of bactericidal activity as a predictor for clinical outcome.
In vitro susceptibility testing of Listeria monocytogenes most often reveals both ampicillin and penicillin as inhibitory as opposed to bactericidal with activity comparable to chloramphenicol and tetracycline. Yet, the former two penicillins are more effective for Listeria meningitis than are the latter agents. Accordingly, we reassessed the bactericidal activity of agents used in listeriosis in order to determine in vitro methodology that would be more predictive of clinical outcome. We found that bactericidal activity for greater than 48 hr by either minimum inhibitory-minimum bactericidal concentration (MIC-MBC) testing or time-kill kinetic studies was the best predictor of clinical efficacy. This correlation may be due to Listeria being a slow-growing microorganism. In addition to ampicillin and penicillin, we found trimethoprim-sulfamethoxazole, vancomycin, and imipenem to exhibit bactericidal activity for 48 hr. For the first two agents, this is in agreement with the results of clinical experience.
Ampicillin↗
Comparison of the bactericidal activity of clindamycin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group.
Time-kill kinetic methodology was used to evaluate the bactericidal activity of cefoxitin, cefotetan, clindamycin, and metronidizole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group. Overall, metronidazole was the most bactericidal agent, with all isolates being killed with less than or equal to 4 micrograms/ml at 24 hr. Clindamycin was the next most bactericidal agent, with 20 of 26 isolates being killed with less than 16 micrograms/ml. Six isolates with clindamycin MICs greater than or equal to 64 micrograms/ml were not killed at 24 hr, with concentrations as high as 256 micrograms/ml. Cefoxitin and cefotetan were the least bactericidal agents tested. Seven isolates with MICs of greater than or equal to 64 micrograms/ml to each agent demonstrated a lack of killing at 24 hr, with concentrations of the respective agent as high as 256 micrograms/ml. At concentrations with either agent of 32 micrograms/ml, the remaining 19 isolates were killed at 24 hr. Of the six B. fragilis isolates resistant to clindamycin, four were also resistant to both cefoxitin and cefotetan. We conclude that in hospitals with cefoxitin-resistant B. fragilis group isolates, metronidazole would provide appropriate therapy.
Bacteroides↗
Inhibitory and bactericidal activity of selected beta-lactam agents alone and in combination with beta-lactamase inhibitors compared with that of cefoxitin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group.
The inhibitory activity of five beta-lactam agents, alone and in combination with a beta-lactamase inhibitor, was compared with that of cefoxitin and metronidazole against 300 beta-lactamase producing Bacteroides fragilis group isolates. Each of the beta-lactamase inhibitors significantly potentiated the activity of the respective beta-lactam. In the presence of clavulanate, the MIC90 (minimum inhibitory concentration) values of amoxicillin and ticarcillin were reduced 64-fold and 32-fold, respectively. Similarly, sulbactam enhanced the activity of ampicillin and cefoperazone 16-fold and 8-fold, respectively, whereas tazobactam potentiated the activity of piperacillin 16-fold. Few strains were resistant to the beta-lactam-beta-lactamase inhibitor combinations and were comprised of strains of B. fragilis, B. thetaiotamicron, and B. distasonis. Of the strains, 7% were resistant to cefoxitin, and none to metronidazole. Using time-kill kinetic studies, the bactericidal activity of the various beta-lactam agents, with and without beta-lactamase inhibitors, was determined and compared with that of cefoxitin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the B. fragilis group. Overall, metronidazole was the most bactericidal agent with all isolates being killed with less than or equal to 4 micrograms/ml at 24 hr. Ampicillin-sulbactam was the next most bactericidal agent with all isolates being killed with less than or equal to 16/8 micrograms/ml of ampicillin-sulbactam at 24 hr. Amoxicillin-clavulanate and cefoperazone-sulbactam had bactericidal activity similar to that of ampicillin-sulbactam. Piperacillin-tazobactam and ticarcillin-clavulanate were bactericidal at higher concentrations with all isolates killed with 64 micrograms/ml of piperacillin and 128 micrograms/ml of ticarcillin combined with their respective beta-lactamase inhibitors. None of the beta-lactam agents alone was able to kill more than 19 of the 26 isolates. We conclude that beta-lactam agents combined with beta-lactamase inhibitors have both inhibitory and bactericidal activity against cefoxitin-resistant members of the B. fragilis group provided that the concentrations achieved for these combinations are at the upper limits for maximum recommended dosing. Although isolates of the B. fragilis group have been reported to produce unusual beta-lactamases that are refractory to beta-lactamase inhibitors, none of the cefoxitin-resistant isolates tested in this study were resistant to the beta-lactam-beta-lactamase inhibitor combinations.
Anti-Bacterial Agents↗
RP 59500, a new streptogramin highly active against recent isolates of North American staphylococci.
To assess the potential clinical utility of RP 59500, 10 investigators from separate locations in the United States and Canada each tested approximately 200 current isolates of staphylococci (Staphylococcus aureus and coagulase-negative staphylococci) by a standard protocol. RP 59500 was highly active (MIC90 < or = 2 micrograms/ml) against all strains, including those that were resistant to oxacillin, ciprofloxacin, erythromycin, and spiramycin.
Coagulase↗
In vitro killing of penicillin-susceptible, -intermediate, and -resistant strains of Streptococcus pneumoniae by cefotaxime, ceftriaxone, and ceftizoxime: a comparison of bactericidal and inhibitory activity with achievable CSF levels.
This study assessed total microbial killing of 30 penicillin-susceptible, -intermediate, and -resistant strains of Streptococcus pneumoniae by cefotaxime, ceftriaxone, and ceftizoxime and compared these values with MICs for each strain against each agent as determined by three different methods/media. The results confirm the appropriateness of recent NCCLS recommendations for MIC interpretive criteria for third generation cephalosporins in which < or = 0.25 microgram/ml = susceptible and > or = 2.0 micrograms/ml = resistant when these agents are used to treat pneumococcal meningitis and data from total microbial killing studies suggests that most isolates with MICs of 0.5 and 1.0 mcg/ml would respond to high dose therapy with all three agents. The study also confirmed the recently described two- to four-fold decrease in activity of ceftizoxime against S. pneumoniae as compared with either cefotaxime or ceftriaxone; but noted that current NCCLS MIC interpretive criteria for the therapy of meningitis remain valid for all three agents. Finally, the study found that MICs determined by the E test or by microdilution broth methods using supplemented Todd Hewitt broth predict susceptibility as well as the NCCLS reference method. The actual selection among these agents for the therapy of pneumococcal meningitis should also consider other parameters including protein binding, age groups of clinical use, maximum potency against all clinically relevant pathogens, and cost.
Cefotaxime↗
Topics in clinical microbiology: tracheostomy and endotracheal tube specimens.
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Bacteria↗
Endemic staphylococcal pseudobacteremia.
Endemic staphylococcal pseudobacteremia is defined as a situation in which coagulase-negative staphylococci are isolated from blood cultures more frequently than would be expected. Such false-positive blood cultures may influence patient care significantly and add considerably to its cost. This appears to be a more common phenomenon in hospitals in which blood cultures are drawn by house staff, but many occur in any hospital. A survey of all positive blood cultures will identify this problem. Careful attention to the proper procedure for collecting blood cultures should help prevent this phenomenon of endemic coagulase-negative staphylococcal pseudobacteremia.
Bacteriological Techniques↗
Nursery epidemic due to multiply-resistant Klebsiella pneumoniae: epidemiologic setting and impact on perinatal health care delivery.
Gram-negative bacilli frequently cause epidemics in high-risk newborn intensive care units. Recently, an epidemic caused by a multiply-resistant K. pneumoniae, serotype 21, occurred in the Vanderbilt University intensive care nursery. The background of this outbreak included an increasing endemic nosocomial sepsis rate, operation of the facility in excess of rated capacity, and increasingly inadequate nurse-to-patient staffing ratios. The epidemic lasted 11 weeks; 26 (12%) of the 232 infants at risk in the unit became colonized. Five infants developed systemic illness and one died. Cohorting, reinforcement of strict handwashing and isolation procedures, and closure of the unit to outborn admissions resulted in rapid termination of the outbreak. Followup studies performed on infants colonized with the epidemic bacterium demonstrated persistent fecal shedding up to 13 months following discharge from the hospital. This epidemic had a detrimental influence on high-risk newborn and obstetric health care delivery in an area encompassing portions of three states. Under a system of progressively more sophisticated referral units, nosocomial infections occurring at a tertiary center can have an impact on other hospitals within the network.
Carrier State↗
Pseudomonas aeruginosa.
P. aeruginosa is widely distributed in nature and in the hospital environment with a predilection for moist areas. Its inherent resistance to many antimicrobials and its ability to produce many enzymes contribute to its pathogenic potential as both a primary and a secondary cause of infection. It is easily grown and identified in the microbiology laboratory. However, susceptibility testing remains a problem. Currently, the best approach to treatment is an aminoglycoside and an antipseudomonal beta-lactam antimicrobial. Typing can differentiate strains, but should be reserved for specific epidemiologic problems.
Anti-Bacterial Agents↗
Epidemiological spectrum and current treatment of listeriosis.
To reassess the epidemiology and treatment of listeriosis in the United States, we reviewed greater than 120 cases of listeriosis from four medical centers in three geographically separated cities: Los Angeles County-University of Southern California Medical Center (LAC-USCMC); Rush-Presbyterian-St. Luke's Hospital, Chicago; the University of Illinois Hospital, Chicago; and Vanderbilt University Hospital, Nashville, Tennessee. The epidemiological pattern at LAC-USCMC was relatively narrow; more than two-thirds of the cases occurred during the perinatal period. Cases at Vanderbilt University Hospital represented the opposite end of the spectrum; the majority of these occurred in nonpregnant, older adults who had received organ transplants. An intermediate pattern of cases was observed at the two medical centers in Chicago. Potential risk factors included pregnancy, neonatal status, organ transplantation, renal failure, malignancy, systemic lupus erythematosus, steroid therapy, and AIDS (two cases). Antimicrobial agents noted to be effective were, as expected, penicillin and ampicillin; the cephalosporins were ineffective. The mortality associated with listeriosis occurred mainly among premature infants and stillbirths delivered from infected pregnant women and was markedly less among neonates and adults.
Adult↗
Mechanisms of bacterial resistance to antimicrobial agents.
The 20th century has been considered the antimicrobial era--whereas the 21st century may well represent the post-antimicrobial era. The reason for this dramatic change, should it come to pass, is the development of bacterial resistance to antimicrobial agents. This emerging resistance is now challenging the clinical utility of many antimicrobial agents such that the chemotherapy of hospitalized patients with serious infections has been compromised. If the problem with resistance is to be successfully dealt with by clinicians, the mechanisms of such resistance must be known and understood. This paper thus reviews the most important mechanisms of resistance as well as some of the most important pathogens having these mechanisms. An understanding of these important microbial resistance mechanisms will help the clinician identify circumstances in which resistance may be a problem as well as evaluating the potential usefulness of an alternate antimicrobial agent against resistant microbes.
Anti-Bacterial Agents↗
Coagulase-positive staphylococci.
S. aureus is a frequent and constant colonizer of the skin and mucosal surfaces of humans. It has the capability of producing a large number of enzymes that contribute to its pathogenic potential. S. aureus is a common cause of nosocomial infections, particularly wound- and IV-catheter-associated infections. Lysogenization of these organisms appears to contribute to a number of properties, the most important of which is the resistance to antimicrobials. Such resistance is an important problem--both clinically and in the microbiology laboratory--where the not infrequent inability to identify such resistant strains may contribute to morbidity and mortality of staphylococcal disease.
Anti-Bacterial Agents↗