Susceptibility of 114 isolates of the Bacteroides fragilis group to imipenem and eight other antimicrobial agents

The minimal inhibitory concentrations (MICs) of the newer beta-lactam antibiotic, imipenem, were compared with those of cefoxitin, cefotetan, penicillin, amoxycillin, ticarcillin and metronidazole against 114 clinical isolates of the Bacteroides fragilis group of anaerobic organisms. The ability of clavulanic acid, a beta-lactamase inhibitor, to potentiate the in vitro activity of amoxycillin and ticarcillin was also studied. Using an agar dilution technique we found imipenem to be the most active beta-lactam antibiotic tested having an MIC50 of 0.25 microgram/ml and inhibiting all isolates at a concentration of 4 micrograms/ml. Metronidazole had comparable activity with a MIC50 of 0.5 microgram/ml and all isolates inhibited by 1 microgram/ml. Cefoxitin and cefotetan showed similar activity both with a MIC50 of 8 micrograms/ml against the B. fragilis group, while penicillin, amoxycillin and ticarcillin all had a MIC50 of 16 micrograms/ml. Clavulanic acid significantly reduced the MIC50 of amoxycillin and ticarcillin to 0.5 micrograms/ml and 0.25 micrograms/ml, respectively.     

Susceptibility ofLegionella spp. to imipenem and 27 other beta-lactam antibiotics

With 28 beta-lactam antibiotics the susceptibilities of 60 strains ofLegionella spp. (49Legionella pneumophila and 11 ATCC type strains of otherLegionella species) were determined. Agar dilution testing was used on buffered charcoal-yeast extract agar to which 0.1% α-ketoglutarate was added. The most active of the penicillins tested were ampicillin (MIC=0.06–8mg/l) and temocilin (MIC=0.125–16 mg/l); the most active cephalosporins were ceftazidime (MIC=0.03–0.5 mg/l), HR 810 (MIC=0.06?1 mg/l) and cefoxitin (MIC=0.125 – 2 mg/l). Of all the drugs tested imipenem had the most pronounced activity (MIC=.0075 – 0.06 mg/l).     

Susceptibility of anaerobic bacteria to antimicrobial agents

The antimicrobial susceptibility of 1,117 clinical isolates of anaerobic bacteria was determined by the agar dilution technique. Metronidazole was the most active agent; only Propionibacterium acnes and Actinomyces sp. isolates were resistant. Clindamycin and chloramphenical were the next most effective agents. Beta-lactam antibiotics, with the exception of penicillin, were active against most anaerobes other than the Bacteroides fragilis group. At a breakpoint of 8 mg/l, 25% of Fusobacterium spp. and 30% of the non-fragilis Bacteroides spp. were resistant to penicillin. The highest resistance to beta-lactams was seen in the B. fragilis group. Within the indole-positive members of the group, resistance rates of 71% were seen for cefoxitin, 49% for moxalactam, 79% for cefotaxime, 22% for piperacillin and 89% for penicillin. We conclude that metronidazole has the most predictable in vitro activity against common clinical anaerobic isolates and that resistance to beta-lactams was frequent and of potential clinical importance as these latter agents are frequently used in the prophylaxis and therapy of mixed anaerobic infections.     

Susceptibility of Campylobacter jejuni to twenty-three antimicrobial agents

Twenty-three antimicrobial agents, including 4 new broadspectrum beta-lactam antibiotics were tested against 50 clinical isolates of Campylobacter jejuni. The activity of metabolites of metronidazole and tinidazole was also tested. Minimum inhibitory concentrations (MIC) were determined by agar dilution. beta-lactamase production was detected by a chromogenic cephalosporin method. All strains were susceptible to erythromycin, clindamycin, chloramphenicol, furazolidone, aminoglycosides (including gentamicin), cefotaxime and NF-thienamycin. All isolates were resistant to penicillin, cephalothin, cefoperazone, vancomycin, rifampicin and trimethoprim; beta-lactamase was detected in 2% of isolates. Some strains were resistant and others sensitive to the other drugs tested, which included ampicillin, moxalactum tetracycline, metronidazole and tinidazole. The 'hydroxy' metabolites of metronidazole and tinidazole were more active than the parent compounds.     

Susceptibility of Erysipelothrix rhusiopathiae to antimicrobial agents and home disinfectants

AIM: Erysipelothrix rhusiopathiae causes the occupationally-related infection erysipeloid in humans, and may be responsible for infections in lobster fishermen in Western Australia. There are little recent data pertaining to antimicrobial susceptibility, or susceptibility to disinfectants that might be used in the environment. The aim of this study was to determine the susceptibility of E. rhusiopathiae from human, animal and environmental sources to various antimicrobial agents and disinfectants. METHODS: The susceptibility of 60 E rhusiopathiae isolates was determined using a recommended agar dilution procedure. Susceptibility to disinfectants was achieved using a broth microdilution method. RESULTS: Penicillin and ceftriaxone, with low minimum inhibitory concentrations (MICs) (MIC90 0.03 mg/l and 0.125 mg/l, respectively), remained active against E. rhusiopathiae and should continue to be recommended for treatment. Ciprofloxacin MICs were particularly low (MIC90 0.06 mg/l), offering an alternative agent for the penicillin allergic patient. Erysipelothrix rhusiopathiae is still resistant to vancomycin (MIC90 64 mg/l), highlighting the importance of early diagnosis of E. rhusiopathiae infection in cases of endocarditis. In addition, 31 E. rhusiopathiae isolates were tested against several commercially available home disinfectants. Most were effective in killing E. rhusiopathiae with minimum bactericidal concentrations of 0.001% for Pine O Cleen, and 0.03% for Domestos, Linely and the Wheelie Bin Phenyl Cleanser. CONCLUSIONS: There appeared to be no new emergence of antibiotic resistance in E. rhusiopathiae. Various disinfectants could be used following mechanical cleaning of work environments, such as fishing boats, and equipment, to reduce the risk of infection with E. rhusiopathiae.     

Susceptibility of archaea to antimicrobial agents: applications to clinical microbiology

We herein review the state of knowledge regarding the in vitro and in vivo susceptibility of archaea to antimicrobial agents, including some new molecules. Indeed, some archaea colonizing the human microbiota have been implicated in diseases such as periodontopathy. Archaea are characterized by their broad-spectrum resistance to antimicrobial agents. In particular, their cell wall lacks peptidoglycan, making them resistant to antimicrobial agents interfering with peptidoglycan biosynthesis. Archaea are, however, susceptible to the protein synthesis inhibitor fusidic acid and imidazole derivatives. Also, squalamine, an antimicrobial agent acting on the cell wall, proved effective against human methanogenic archaea. In vitro susceptibility data could be used to design protocols for the decontamination of complex microbiota and the selective isolation of archaea in anaerobic culture.     

Susceptibility of clinical isolates ofCampylobacter pylori to twenty-one antimicrobial agents

The MICs of 21 antimicrobial agents were determined for 97 clinical isolates ofCampylobacter pylori. The beta-lactams (penicillin, ampicillin, cefoxitin and cephalexin), macrolides (erythromycin and azithromycin), quinolones (ciprofloxacin and ofloxacin), nitrofurans, gentamicin and tetracycline all had MIC90 values of 0.5 mg/l. Aztreonam, flucloxacillin, amifloxacin and rifampicin had moderate activity. All isolates were resistant to vancomycin, cefsulodin and amphotericin B. Five percent of the strains were inhibited by 8 mg/l of polymyxin. Of the oral agents tested, the nitrofurans and ampicillin are probably the most appropriate antimicrobial agents. Azithromycin and the oral form of cefuroxime are promising alternatives. Cefsulodin, vancomycin and amphotericin B would be suitable constituents of selective media for isolation ofCampylobacter pylori.     

Susceptibility to antimicrobial agents ofPseudomonas aeruginosa attached to siliconized latex urinary catheters

The effect of siliconized latex urinary catheters on the in vitro activity of amikacin, ceftazidime, ciprofloxacin, norfloxacin and meropenem againstPseudomonas aeruginosa was determined by a microdilution assay. MICs of amikacin and meropenem increased at least 4-fold and 16-fold respectively in the presence of catheter material. The effect of catheter material on meropenem activity was not strain dependent and was similar for different brands of catheters. The susceptibility to antimicrobial agents ofPseudomonas aeruginosa attached to catheters for 6 and 24 hours was also evaluated. When bacteria attached for 6 hours were used as inoculum, MBCs increased at least 8-fold for amikacin, 64-fold for ceftazidime, 64-fold for ciprofloxacin, 32-fold for norfloxacin and 2048-fold for meropenem. Similar results were observed when bacteria attached to catheters for 24 hours were used as inoculum. It is concluded that catheter material itself affected the in vitro activity of meropenem, and that the bactericidal activity of all antimicrobial agents againstPseudomonas aeruginosa present in biofilms on the surface of siliconized latex urinary catheters decreased dramatically, this effect being more pronounced with meropenem.     

Susceptibility of Neisseria meningitidis to 16 antimicrobial agents and characterization of resistance mechanisms affecting some agents

Neisseria meningitidis represents a pathogen of great public health importance in both developed and developing countries. Resistance to some antimicrobial agents used either for therapy of invasive infections or for prophylaxis of case contacts has long been recognized, although specific guidelines for susceptibility testing have not been fully developed. We have examined the susceptibilities of a collection of 442 meningococcal clinical isolates from 15 countries to 16 antimicrobial agents. These included isolates recovered between 1917 and 2004, with representatives of all major serogroups. All isolates were tested by the Clinical and Laboratory Standards Institute (formerly NCCLS) broth microdilution method using Mueller-Hinton lysed horse blood broth, while a subset of 102 isolates was tested by agar dilution using Mueller-Hinton sheep blood agar. Most isolates provided adequate growth for MIC determinations by both broth and agar methods. Growth in broth was enhanced by CO(2) incubation and was required for two strains (1.7%). MICs of the study drugs compared favorably between the broth and agar methods (79 to 100% essential agreement), and MICs also generally agreed closely (92 to 100% essential agreement, excluding azithromycin) between broth tests incubated in the two different atmospheres. Elevated penicillin and ampicillin MICs (> or =0.12 microg/ml and > or =0.25 microg/ml, respectively) occurred in 14.3% and 8.6% of strains and were associated with polymorphisms of the penA gene encoding a modified penicillin-binding protein 2. None of the 442 isolates produced beta-lactamase. Elevated tetracycline and doxycycline (but not minocycline) MICs were associated with efflux-mediated resistance encoded by tet(B) in 13 strains. Resistance to sulfisoxazole in 21.7% of strains and to trimethoprim-sulfamethoxazole in 21.0% resulted from polymorphisms of folP encoding a modified dihydropteroate synthetase. Seven strains were resistant to rifampin due to mutations in the rpoB gene, and two strains were resistant to chloramphenicol due to production of chloramphenicol acetyltransferase mediated by catP. Two strains had reduced quinolone susceptibility due to mutations of gyrA. The determination of the susceptibilities of a large group of meningococcal strains (including strains with characterized resistance mechanisms) to 16 antimicrobial agents has served as the essential first step in defining susceptibility testing breakpoints specific for this organism.     

Inability to adequately control antimicrobial agents on AutoMicrobic System Gram-Positive and Gram-Negative Susceptibility Cards.

The Vitek Gram-Positive Susceptibility Card (GPS) and Gram-Negative General Susceptibility Plus Card (GSC Plus) were tested on the AutoMicrobic System (AMS) 50 times each with the recommended control organisms. Only 1 drug (chloramphenicol) of 11 on the GPS and 1 (gentamicin) of 10 on the GSC Plus could be adequately controlled, leaving unsubstantiated the results obtained with patient isolates on the remaining 19 antimicrobial agents.     

Susceptibility ofMycobacterium kansasii to ethambutol and its combination with rifamycins,ciprofloxacin and isoniazid

The susceptibility ofMycobacterium kansasii to antibacterial agents alone and in combination was studied. Widespread resistance to ethambutol, ciprofloxacin and isoniazid was found when these drugs were tested separately. However, pronounced antibacterial effects were seen when ethambutol was tested in combination with ciprofloxacin, rifampicin or rifabutin, which corresponded to significantly decreased resistance to these drugs in combination.     

Susceptibility to antimicrobial agents and analysis of plasmids in gentamicin- and methicillin-resistant Staphylococcus aureus from Dublin hospitals

Methicillin- and gentamicin-resistant Staphylococcus aureus (MGRSA) strains isolated from Dublin Hospitals were classified into two groups (phenotypes). Phenotype-I strains expressed high level resistance to gentamicin and were susceptible to fusidic acid; strains resistant to tetracycline harboured a 3 X 10(6)-mol. wt plasmid. Strains in phenotype II usually expressed low level resistance to gentamicin, were resistant to fusidic acid and often harboured a (22-24) X 10(6)-mol. wt plasmid that specified resistance to ethidium bromide, tetracycline, kanamycin, neomycin and trimethoprim, or to combinations of these markers. A few phenotype-II strains expressed higher levels of resistance to gentamicin and other aminoglycosides. All MGRSA strains carried a 21 X 10(6)-mol. wt plasmid conferring resistance to penicillin, ethidium bromide, cadmium and mercury. Gentamicin resistance was invariably chromosomal and all strains carried chromosomal resistance to methicillin, erythromycin, streptomycin and spectinomycin. Several methicillin-resistant S. aureus (MRSA) strains isolated before the emergence of gentamicin resistance harboured a 21 X 10(6)-mol. wt penicillinase plasmid with the same restriction endonuclease profile as that from some MGRSA strains. Some MRSA strains carried other plasmids related to those found in MGRSA strains.     

A simplified method for testing Bordetella pertussis for resistance to erythromycin and other antimicrobial agents

Present methods of antimicrobial susceptibility testing of Bordetella pertussis are time consuming and require specialized media that are not commercially available. We tested 52 isolates of B. pertussis for resistance to erythromycin, trimethoprim-sulfamethoxazole, chloramphenicol, and rifampin by agar dilution with Bordet-Gengou agar (BGA) containing 20% horse blood (reference method), Etest using BGA and Regan-Lowe agar without cephalexin (RL-C), and disk diffusion using BGA and RL-C. The organisms tested included four erythromycin-resistant isolates of B. pertussis from a single patient, a second erythromycin-resistant strain of B. pertussis from an unrelated patient in another state, and 47 nasopharyngeal surveillance isolates of B. pertussis from children in the western United States. The results of agar dilution testing using direct inoculation of the organisms suspended in Mueller-Hinton broth were within +/-1 dilution of those obtained after overnight passage of the inoculum in Stainer-Scholte medium, which is the traditional method of testing B. pertussis. The Etest method produced MICs similar to those of the agar dilution reference method for three of the four antimicrobial agents tested; the trimethoprim-sulfamethoxazole results were lower with Etest, particularly when the direct suspension method was used. Most of the Etest MICs, except for that of erythromycin, were on scale. Disk diffusion testing using RL-C medium was helpful in identifying the erythromycin-resistant strains, which produced no zone of inhibition around the disk; susceptible isolates produced zones of at least 42 mm. Thus, the antimicrobial susceptibility testing of B. pertussis can be simplified by using the Etest or disk diffusion on RL-C to screen for erythromycin-resistant isolates of B. pertussis.     

Susceptibility of Arcobacter butzleri, Arcobacter cryaerophilus, and Arcobacter skirrowii to antimicrobial agents used in selective media

Several antimicrobial agents used in selective media for the isolation of Arcobacter were found to be inhibitory to strains belonging to this genus. All three species tested were susceptible to colistin and rifampin at concentrations used in selective media. Arcobacter skirrowii was the most susceptible species. 5-Fluorouracil, novobiocin, trimethoprim, and teicoplanin or vancomycin were found to be without any inhibitory effect on the strains tested at concentrations described for the isolation of Arcobacter species.