Medium compositions and culture conditions for the assay of fosfomycin susceptibility by Etest

The Etest (AB Biodisk, Solna, Sweden), a susceptibility testing method, was used for fosfomycin and was evaluated by comparison with the agar dilution method for 73 clinical isolates of Escherichia coli, including two resistant strains, and an optimal Etest method for fosfomycin was determined. Media and culture conditions greatly affected the minimum inhibitory concentrations (MICs) determined by the Etest for fosfomycin, as shown for the agar dilution methods. Our results showed that the most favorable conditions for the Etest for fosfomycin were with Mueller-Hinton agar (Becton-Dickinson Japan, Tokyo, Japan) under aerobic conditions. However, the MICs for the resistant strains were much higher than those determined by agar dilution methods, using Nutrient agar (Becton-Dickinson Japan) under anaerobic conditions. The addition of glucose-6-phosphate did not significantly affect the Etest results. Received: August 5, 1999 / Accepted: October 28, 1999     

Antimicrobial Activity of Fosfomycin-Tobramycin Combination against Pseudomonas aeruginosa Isolates Assessed by Time-Kill Assays and Mutant Prevention Concentrations

The antibacterial activity of fosfomycin-tobramycin combination was studied by time-kill assay in eight Pseudomonas aeruginosa clinical isolates belonging to the fosfomycin wild-type population (MIC = 64 μg/ml) but with different tobramycin susceptibilities (MIC range, 1 to 64 μg/ml). The mutant prevention concentration (MPC) and mutant selection window (MSW) were determined in five of these strains (tobramycin MIC range, 1 to 64 μg/ml) in aerobic and anaerobic conditions simulating environments that are present in biofilm-mediated infections. Fosfomycin-tobramycin was synergistic and bactericidal for the isolates with mutations in the mexZ repressor gene, with a tobramycin MIC of 4 μg/ml. This effect was not observed in strains displaying tobramycin MICs of 1 to 2 μg/ml due to the strong bactericidal effect of tobramycin alone. Fosfomycin presented higher MPC values (range, 2,048 to >2,048 μg/ml) in aerobic and anaerobic conditions than did tobramycin (range, 16 to 256 μg/ml). Interestingly, the association rendered narrow or even null MSWs in the two conditions. However, for isolates with high-level tobramycin resistance that harbored aminoglycoside nucleotidyltransferases, time-kill assays showed no synergy, with wide MSWs in the two environments. glpT gene mutations responsible for fosfomycin resistance in P. aeruginosa were determined in fosfomycin-susceptible wild-type strains and mutant derivatives recovered from MPC studies. All mutant derivatives had changes in the GlpT amino acid sequence, which resulted in a truncated permease responsible for fosfomycin resistance. These results suggest that fosfomycin-tobramycin can be an alternative for infections due to P. aeruginosa since it has demonstrated synergistic and bactericidal activity in susceptible isolates and those with low-level tobramycin resistance. It also prevents the emergence of resistant mutants in either aerobic or anaerobic environments.     

Pharmacodynamics of Fosfomycin: Insights into Clinical Use for Antimicrobial Resistance

The aim of this study was to improve the understanding of the pharmacokinetic-pharmacodynamic relationships of fosfomycin against extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli strains that have different fosfomycin MICs. Our methods included the use of a hollow fiber infection model with three clinical ESBL-producing E. coli strains. Human fosfomycin pharmacokinetic profiles were simulated over 4 days. Preliminary studies conducted to determine the dose ranges, including the dose ranges that suppressed the development of drug-resistant mutants, were conducted with regimens from 12 g/day to 36 g/day. The combination of fosfomycin at 4 g every 8 h (q8h) and meropenem at 1 g/q8h was selected for further assessment. The total bacterial population and the resistant subpopulations were determined. No efficacy was observed against the Ec42444 strain (fosfomycin MIC, 64 mg/liter) at doses of 12, 24, or 36 g/day. All dosages induced at least initial bacterial killing against Ec46 (fosfomycin MIC, 1 mg/liter). High-level drug-resistant mutants appeared in this strain in response to 12, 15, and 18 g/day. In the study arms that included 24 g/day, once or in a divided dose, a complete extinction of the bacterial inoculum was observed. The combination of meropenem with fosfomycin was synergistic for bacterial killing and also suppressed all fosfomycin-resistant clones of Ec2974 (fosfomycin MIC, 1 mg/liter). We conclude that fosfomycin susceptibility breakpoints (≤64 mg/liter according to CLSI [for E. coli urinary tract infections only]) should be revised for the treatment of serious systemic infections. Fosfomycin can be used to treat infections caused by organisms that demonstrate lower MICs and lower bacterial densities, although relatively high daily dosages (i.e., 24 g/day) are required to prevent the emergence of bacterial resistance. The ratio of the area under the concentration-time curve for the free, unbound fraction of fosfomycin versus the MIC (fAUC/MIC) appears to be the dynamically linked index of suppression of bacterial resistance. Fosfomycin with meropenem can act synergistically against E. coli strains in preventing the emergence of fosfomycin resistance.     

In Vitro Activity and the Efficacy of Arbekacin,Cefminox, Fosfomycin,Biapenem Against Gram-Negative Organisms: New Treatment Options?

In recent years bacteria have shown much more potential to develop antimicrobial resistance. This mandates not only to the development of new drugs but also to the re-evaluation of already existing ones. Present study is sought to evaluate the in vitro activity of a few newer antibiotics which are not marketed in many countries at present. This study evaluates the in vitro activity of newer antibiotics i. e arbekacin, cefminox, fosfomycin and biapenem against clinical isolates Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. The minimum inhibition concentration of antibiotics was determined by the agar dilution method. The main finding of the present study is that fosfomycin was active against both E. coli and K. pneumoniae where as cefminox exhibited an insignificant intermediate level activity against both E. coli and K. pneumoniae. Biapenem was more active on E. coli than other tested organisms. Colistin showed inhibition for the entire organisms in present study. The present study suggests that colistin and fosfomycin are promising antibiotics for future use; however clinical trials are needed to confirm these findings and to evaluate potency of these four antibiotics.     

Antibiotic Susceptibility of Enterohemorrhagic Escherichia coli O157:H7 Isolated from an Outbreak in Japan in 1996

The antibiotic susceptibilities of 43 strains of Escherichia coli O157:H7 identified in the summer of 1996 in Japan were investigated. Growth of 90% of O157 strains was inhibited at a concentration of ≤0.5 μg/ml by several agents including fosfomycin with glucose-6-phosphate.     

Exploration of the Pharmacokinetic-Pharmacodynamic Relationships for Fosfomycin Efficacy Using an In Vitro Infection Model

Fosfomycin, a phosphonic class antibiotic with a broad spectrum of antibacterial activity, has been used outside the United States since the early 1970s for the treatment of a variety of infections. In the United States, an oral (tromethamine salt) formulation is used for uncomplicated urinary tract infections. Recently, there has been interest in the use of an intravenous solution (ZTI-01) for the treatment of a broad range of infections associated with multidrug-resistant bacteria. In this era of multidrug-resistant bacteria with few treatment options, it is critical to understand the pharmacokinetic-pharmacodynamic (PK-PD) determinants for fosfomycin efficacy. Since such data are limited, a one-compartment in vitro infection model was used to determine the PK-PD index associated with efficacy and the magnitude of this measure necessary for various levels of effect. One challenge isolate (Escherichia coli ATCC 25922, for which the fosfomycin agar MIC is 0.5 mg/liter and the broth microdilution MIC is 1 mg/liter) was evaluated in the dose fractionation studies, and two additional clinical E. coli isolates were evaluated in the dose-ranging studies. Mutation frequency studies indicated the presence of an inherently fosfomycin resistant E. coli subpopulation (agar MIC = 32 to 64 mg/liter) within the standard starting inoculum of a susceptibility test. Due to the presence of this resistant subpopulation, we identified the percentage of the dosing interval that drug concentrations were above the inherent resistance inhibitory concentration found at baseline to be the PK-PD index associated with efficacy (r² = 0.777). The magnitudes of this PK-PD index associated with net bacterial stasis and 1- and 2-log₁₀ CFU/ml reductions from baseline at 24 h were 11.9, 20.9, and 32.8, respectively. These data provide useful information for modernizing and optimizing ZTI-01 dosing regimens for further study.     

Clinical Appraisal of Fosfomycin in the Era of Antimicrobial Resistance

Fosfomycin is recommended as one of the first-line agents for treatment of urinary tract infections (UTIs) in the latest guidelines endorsed by the Infectious Diseases Society of America (IDSA) and the European Society for Clinical Microbiology and Infectious Diseases (ESCMID). We evaluated the use of fosfomycin among inpatients at a tertiary care hospital between 2009 and 2013. UTI cases were defined using physician diagnosis and the National Healthcare Safety Network (NHSN) surveillance definitions. The number of patients treated with fosfomycin increased from none in 2009 to 391 in 2013. Among 537 patients who received fosfomycin for any indication during this period, UTI was the most common indication (74%), followed by asymptomatic bacteriuria (10%). All except 19 patients received a single dose of fosfomycin. Escherichia coli was the most common organism involved (52%). For 119 patients with UTIs, after exclusion of those with negative urine culture results, negative urinalysis results, receipt of additional agents, or indeterminate clinical outcomes, the clinical success rate at 48 h was 74.8%. Of 89 patients who met the criteria for NHSN-defined UTIs, 89.9% had successful outcomes. Recurrent infections occurred in 4.3% of cases, and mild adverse events were observed in 2.0%. All 100 randomly selected extended-spectrum β-lactamase (ESBL)-producing E. coli clinical isolates from this period were susceptible to fosfomycin. In conclusion, the use of fosfomycin has increased substantially since implementation of the updated guidelines at this hospital. Fosfomycin was used mainly for the treatment of physician-diagnosed UTIs, and the clinical outcomes were generally favorable. Fosfomycin maintained activity against E. coli despite the increased use of the agent.     

Antibiotic Trapping by Plasmid-Encoded CMY-2 β-Lactamase Combined with Reduced Outer Membrane Permeability as a Mechanism of Carbapenem Resistance in Escherichia coli

A liver transplant patient was admitted with cholangitis, for which meropenem therapy was started. Initial cultures showed a carbapenem-susceptible (CS) Escherichia coli strain, but during admission, a carbapenem-resistant (CR) E. coli strain was isolated. Analysis of the outer membrane protein profiles showed that both CS and CR E. coli lacked the porins OmpF and OmpC. Furthermore, PCR and sequence analysis revealed that both CS and CR E. coli possessed bla_CTX-M-15 and bla_OXA-1. The CR E. coli strain additionally harbored bla_CMY-2 and demonstrated a >15-fold increase in β-lactamase activity against nitrocefin, but no hydrolysis of meropenem was detected. However, nitrocefin hydrolysis appeared strongly inhibited by meropenem. Furthermore, the CMY-2 enzyme demonstrated lower electrophoretic mobility after its incubation either in vitro or in vivo with meropenem, indicative of its covalent modification with meropenem. The presence of the acyl-enzyme complex was confirmed by mass spectrometry. By transformation of the CMY-2-encoding plasmid into various E. coli strains, it was established that both porin deficiency and high-level expression of the enzyme were needed to confer meropenem resistance. In conclusion, carbapenem resistance emerged by a combination of elevated β-lactamase production and lack of porin expression. Due to the reduced outer membrane permeability, only small amounts of meropenem can enter the periplasm, where they are trapped but not degraded by the large amount of the β-lactamase. This study, therefore, provides evidence that the mechanism of “trapping” by CMY-2 β-lactamase plays a role in carbapenem resistance.     

Nationwide surveillance of bacterial pathogens isolated from patients with acute uncomplicated cystitis in 2018: Conducted by the Japanese Research Group for Urinary Tract Infections (JRGU)

IntroductionThe aim of this study was to monitor the development of drug-resistant bacteria isolated from acute uncomplicated cystitis (AUC) and to evaluate methodology of the survey conducted by collecting only clinical data.MethodsWe enrolled female patients at least 16 years of age diagnosed with AUC in 2018. Patient information including age, menopausal status, and results of bacteriological examination were collected and analyzed regardless of bacterial identification, antimicrobial susceptibility testing or extended-spectrum β-lactamase (ESBL) detection method.ResultsA total of 847 eligible cases were collected. Escherichia coli (E. coli) was the most frequently isolated bacterial species at about 70%, with proportions of fluoroquinolone-resistant E. coli (QREC) and ESBL-producing E. coli isolates at 15.6% and 9.5% of all E. coli isolates, respectively. The proportion of Staphylococcus saprophyticus (S. saprophyticus) was significantly higher in premenopausal women. Regarding the drug susceptibility of E. coli, isolates from Eastern Japan had significantly higher susceptibility to cefazolin, cefotiam and cefpodoxime and lower susceptibility to levofloxacin in postmenopausal women. ESBL-producing E. coli isolates had a high susceptibility to tazobactam-piperacillin, cefmetazole, carbapenems, aminoglycosides, and fosfomycin. In S. saprophyticus, the susceptibility to β-lactams including carbapenems was 40–60%.ConclusionsThe proportions of QREC and ESBL-producing E. coli were increasing trends and lower susceptibility to LVFX in postmenopausal women was observed. Such surveillance, consisting of the collecting only clinical data, could be conducted easily and inexpensively. It is expected to be continuously performed as an alternative survey to conventional one collecting bacterial strains.     

fosI Is a New Integron-Associated Gene Cassette Encoding Reduced Susceptibility to Fosfomycin

In this work, we demonstrate that the fosI gene encodes a predicted small protein with 134 amino acids and determines reduced susceptibility to fosfomycin. It raised the MIC from 0.125 to 6 μg/ml when the pBRA100 plasmid was introduced into Escherichia coli TOP10 and to 16 μg/ml when the gene was cloned into the pBC_SK(−) vector and expressed in E. coli TOP10.     

Community-Onset Escherichia coli Infection Resistant to Expanded-Spectrum Cephalosporins in Low-Prevalence Countries

By global standards, the prevalence of community-onset expanded-spectrum-cephalosporin-resistant (ESC-R) Escherichia coli remains low in Australia and New Zealand. Of concern, our countries are in a unique position, with high extramural resistance pressure from close population and trade links to Asia-Pacific neighbors with high ESC-R E. coli rates. We aimed to characterize the risks and dynamics of community-onset ESC-R E. coli infection in our low-prevalence region. A case-control methodology was used. Patients with ESC-R E. coli or ESC-susceptible E. coli isolated from blood or urine were recruited at six geographically dispersed tertiary care hospitals in Australia and New Zealand. Epidemiological data were prospectively collected, and bacteria were retained for analysis. In total, 182 patients (91 cases and 91 controls) were recruited. Multivariate logistic regression identified risk factors for ESC-R among E. coli strains, including birth on the Indian subcontinent (odds ratio [OR] = 11.13, 95% confidence interval [95% CI] = 2.17 to 56.98, P = 0.003), urinary tract infection in the past year (per-infection OR = 1.430, 95% CI = 1.13 to 1.82, P = 0.003), travel to southeast Asia, China, the Indian subcontinent, Africa, and the Middle East (OR = 3.089, 95% CI = 1.29 to 7.38, P = 0.011), prior exposure to trimethoprim with or without sulfamethoxazole and with or without an expanded-spectrum cephalosporin (OR = 3.665, 95% CI = 1.30 to 10.35, P = 0.014), and health care exposure in the previous 6 months (OR = 3.16, 95% CI = 1.54 to 6.46, P = 0.02). Among our ESC-R E. coli strains, the bla_CTX-M ESBLs were dominant (83% of ESC-R E. coli strains), and the worldwide pandemic ST-131 clone was frequent (45% of ESC-R E. coli strains). In our low-prevalence setting, ESC-R among community-onset E. coli strains may be associated with both “export” from health care facilities into the community and direct “import” into the community from high-prevalence regions.     

An increase in the antimicrobial activity in vitro of fosfomycin under anaerobic conditions

Fosfomycin showed lower MIC values under anaerobic culture conditions than those under aerobic conditions for four Gram-positive and 18 Gram-negative bacterial isolates. The degree of change in MICs was dependent on the culture media and strains tested. The growth-inhibitory diameter in the paper disc assay increased in parallel with the decrease in the redox potential of the agar medium. Bacteriolytic activity was also potentiated in anaerobiosis. The increase of the bio-activity of fosfomycin in anaerobic culture was neither due to the change of medium pH nor to the change of mobility of drug in agar. However it is possible that the uptake of fosfomycin through the cell membrane increased in anaerobiosis. The anaerobic MICs of fosfomycin were better correlated than the aerobic MICs with the ED50 values in a mouse systemic infection model.     

An In Vitro Deletion in ribE Encoding Lumazine Synthase Contributes to Nitrofurantoin Resistance in Escherichia coli

Nitrofurantoin has been used for decades for the treatment of urinary tract infections (UTIs), but clinically significant resistance in Escherichia coli is uncommon. Nitrofurantoin concentrations in the gastrointestinal tract tend to be low, which might facilitate selection of nitrofurantoin-resistant (NIT-R) strains in the gut flora. We subjected two nitrofurantoin-susceptible intestinal E. coli strains (ST540-p and ST2747-p) to increasing nitrofurantoin concentrations under aerobic and anaerobic conditions. Whole-genome sequencing was performed for both susceptible isolates and selected mutants that exhibited the highest nitrofurantoin resistance levels aerobically (ST540-a and ST2747-a) and anaerobically (ST540-an and ST2747-an). ST540-a/ST540-an and ST2747-a (aerobic MICs of >64 μg/ml) harbored mutations in the known nitrofurantoin resistance determinants nfsA and/or nfsB, which encode oxygen-insensitive nitroreductases. ST2747-an showed reduced nitrofurantoin susceptibility (aerobic MIC of 32 μg/ml) and exhibited remarkable growth deficits but did not harbor nfsA/nfsB mutations. We identified a 12-nucleotide deletion in ribE, encoding lumazine synthase, an essential enzyme involved in the biosynthesis of flavin mononucleotide (FMN), which is an important cofactor for NfsA and NfsB. Complementing ST2747-an with a functional wild-type lumazine synthase restored nitrofurantoin susceptibility. Six NIT-R E. coli isolates (NRCI-1 to NRCI-6) from stools of UTI patients treated with nitrofurantoin, cefuroxime, or a fluoroquinolone harbored mutations in nfsA and/or nfsB but not ribE. Sequencing of the ribE gene in six intestinal and three urinary E. coli strains showing reduced nitrofurantoin susceptibility (MICs of 16 to 48 μg/ml) also did not identify any relevant mutations. NRCI-1, NRCI-2, and NRCI-5 exhibited up to 4-fold higher anaerobic MICs, compared to the mutants generated in vitro, presumably because of additional mutations in oxygen-sensitive nitroreductases.     

Activities of Fosfomycin,Tigecycline, Colistin,and Gentamicin against Extended-Spectrum-β-Lactamase-Producing Escherichia coli in a Foreign-Body Infection Model

Limited antimicrobial agents are available for the treatment of implant-associated infections caused by fluoroquinolone-resistant Gram-negative bacilli. We compared the activities of fosfomycin, tigecycline, colistin, and gentamicin (alone and in combination) against a CTX-M15-producing strain of Escherichia coli (Bj HDE-1) in vitro and in a foreign-body infection model. The MIC and the minimal bactericidal concentration in logarithmic phase (MBC_log) and stationary phase (MBC_stat) were 0.12, 0.12, and 8 μg/ml for fosfomycin, 0.25, 32, and 32 μg/ml for tigecycline, 0.25, 0.5, and 2 μg/ml for colistin, and 2, 8, and 16 μg/ml for gentamicin, respectively. In time-kill studies, colistin showed concentration-dependent activity, but regrowth occurred after 24 h. Fosfomycin demonstrated rapid bactericidal activity at the MIC, and no regrowth occurred. Synergistic activity between fosfomycin and colistin in vitro was observed, with no detectable bacterial counts after 6 h. In animal studies, fosfomycin reduced planktonic counts by 4 log₁₀ CFU/ml, whereas in combination with colistin, tigecycline, or gentamicin, it reduced counts by >6 log₁₀ CFU/ml. Fosfomycin was the only single agent which was able to eradicate E. coli biofilms (cure rate, 17% of implanted, infected cages). In combination, colistin plus tigecycline (50%) and fosfomycin plus gentamicin (42%) cured significantly more infected cages than colistin plus gentamicin (33%) or fosfomycin plus tigecycline (25%) (P < 0.05). The combination of fosfomycin plus colistin showed the highest cure rate (67%), which was significantly better than that of fosfomycin alone (P < 0.05). In conclusion, the combination of fosfomycin plus colistin is a promising treatment option for implant-associated infections caused by fluoroquinolone-resistant Gram-negative bacilli.     

Sitafloxacin has a potent activity for eradication of extended spectrum β-lactamase-producing fluoroquinolone-resistant Escherichia coli forming intracellular bacterial communities in uroepithelial cells

IntroductionEradication of asymptomatic bacteriuria (ASB) before urological procedures is important to reduce the risk for infectious complications after surgery. However, the appropriate regimen for antimicrobial treatment has not been fully determined. We experienced continuous (over 10 months) isolation of extended spectrum β-lactamase (ESBL)-producing fluoroquinolone-resistant Escherichia coli from urine of an asymptomatic patient. The four isolates obtained (SMESC1 to 4) were international high-risk clones of O25b:H4-ST131-H30R, and originated from one strain, as revealed by the whole genome sequences. Although the patient received meropenem (MEPM) and fosfomycin (FOM), to which the strains were susceptible before the urological procedures, they could not be eradicated.MethodsTo explore the reason for the continuous isolation even after MEPM and FOM administration, antimicrobial killing of adherent and/or intracellular bacterial communities (IBC) formed by coculture of the E. coli cells and T24 bladder epithelial cells were examined.ResultsFOM and levofloxacin did not decrease viable E. coli cells compared with gentamicin. MEPM partly decreased them, and sitafloxacin (STFX) decreased them most potently. These observations indicate that E. coli can survive in the urinary tract under antimicrobial administration, and some antimicrobials such as FOM and MEPM cannot eradicate E. coli in uroepithelial cells. Adhesion on urinary epithelial cells and/or IBC formation might result in continuous isolation from the urinary tract and recurrence of ASB and urinary tract infections.ConclusionsThe present study suggests that STFX is a promising optional agent for the eradication of ESBL-producing fluoroquinolone-resistant E. coli in the urinary tract before urological procedures.     

Protein Microarray On-Demand: A Novel Protein Microarray System

We describe a novel, simple and low-cost protein microarray strategy wherein the microarrays are generated by printing expression ready plasmid DNAs onto slides that can be converted into protein arrays on-demand. The printed expression plasmids serve dual purposes as they not only direct the synthesis of the protein of interest; they also serve to capture the newly synthesized proteins through a high affinity DNA-protein interaction. To accomplish this we have exploited the high-affinity binding (~3–7×10 ⁻¹³ M) of E. coli Tus protein to Ter, a 20 bp DNA sequence involved in the regulation of E. coli DNA replication. In our system, each protein of interest is synthesized as a Tus fusion protein and each expression construct directing the protein synthesis contains embedded Ter DNA sequence. The embedded Ter sequence functions as a capture reagent for the newly synthesized Tus fusion protein. This “all DNA” microarray can be converted to a protein microarray on-demand without need for any additional capture reagent..     

Functional polymeric dialdehyde dextrin network capped mesoporous silica nanoparticles for pH/GSH dual-controlled drug release

Multi-stimulation responsive nanomaterial-based drug delivery systems promise enhanced therapeutic efficacy in cancer therapy. This work examines a smart pH/GSH dual-responsive drug delivery system by using dialdehyde dextrin (DAD) end-capped mesoporous silica nanoparticles (MSNs). Specifically, DAD was applied as a “gatekeeper polymer” agent to seal drug loads inside the mesoporous of MSNs via a pH-sensitive Schiff bond, whereas the formed DAD polymer shells were further cross-linked by GSH-sensitive disulfide bonds. Results revealed that the DAD gatekeeper polymer could tightly close the mesopores of MSNs to control premature drug release under physiological conditions and respond to acidic and GSH conditions to release the trapped drugs. Significantly, fluorescent microscopy observation and cytotoxicity studies indicated that drug-loaded nanoparticles could be rapidly internalized through a passive targeting effect to inhibit cancer growth. Taken together, these polymer-modified pH/GSH dual-responsive MSNs could be used as promising candidates for “on-demand” anticancer drug delivery applications.

A smart pH/GSH dual-responsive drug delivery system by using DAD as a “gatekeeper polymer” to end-cap MSNs via pH-sensitive Schiff bond, whereas DAD polymer shell were cross-linked by GSH-sensitive disulfide bond.     

Comparison of Tetracycline Action on Staphylococcus aureus and Escherichia coli by Microbial Kinetics

Cultures of tetracycline-treated Staphylococcus aureus exhibited monophasic steady-state growth curves similar to that observed for tetracycline-treated Escherichia coli. Apparent growth rate constants of the respective drug-treated cultures showed the same formal dependence on drug concentration, which was linear at a low concentration but asymptotically approached zero at higher concentration levels and implied the saturation of a limited number of receptor sites engaged in microbial protein synthesis. The relative potency of tetracycline action of S. aureus/E. coli was 6.50:1 at 37.5°C and pH 7.05. This is attributed to relative differences in drug permeation and/or binding affinity for biophase receptors in the respective organisms. It is concluded from kinetic dependencies of growth inhibition of the cultures that tetracycline has the same mode of action on S. aureus and E. coli. It is bacteriostatic at concentrations below the minimal inhibitory concentration level but bactericidal at the higher concentration levels.