Effect of CO2 on susceptibilities of anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin

Abstract

The Oxyrase agar dilution method (Oxyrase, Inc., Mansfield, Ohio), which provides an anaerobic environment without added C02, was compared with the reference agar dilution method recommended by the National Committee for Clinical Laboratory Standards (anaerobic chamber with 10%o C02) to test the susceptibilities of 302 gram-negative and gram-positive anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin. For erythromycin, the overall MIC for 50%6 of isolates tested (MIC50) was 0.5 p,g/ml and the MIC90 was 8.0 ,ug/ml by the Oxyrase method, whereas they were 4.0 and 64.0 ,ug/ml, respectively, under standard anaerobic conditions with CO2. At a breakpoint of 4.0 ,ug/ml, 88% of strains were susceptible to erythromycin by the Oxyrase method, whereas 63% were susceptible in the chamber. The corresponding MIC50s and MIC90s of azithromycin, clarithromycin, and roxithromycin by the Oxyrase method were 0.5 and 8.0, 0.25 and 4.0, and 0.5 and 16.0 ,ug/ml, respectively, whereas in the chamber they were 4.0 and >64.0, 2.0 and 64.0, and 2.0 and 64.0 ug/ml, respectively. At a breakpoint of 8.0 ,ug/ml for these three drugs, 89, 92, and 85% of the isolates, respectively, were susceptible by the Oxyrase method, whereas 67%, 72, and 68% of the isolates, respectively, were susceptible in the chamber. Most strains resistant to all four compounds by both methods were Bacteroides distasonis, Fusobacterium mortiferum, Fusobacterium varium and non-Clostridium perfiringens Clostridium species. Results of the study may lead to a reappraisal of the role played by macrolides and azalides in the treatment of anaerobic infections.Susceptibility testing of macrolides and azalides in the presence of CO2 leads to elevated MICs because of a C02-dependent decrease in the pH of the medium (2,3,14,35,39). Because MICs for anaerobes are determined in an atmosphere of N2, H2, and CO2 in an anaerobic chamber or anaerobic jars, it has generally been assumed that these compounds have low levels of activity against this class of organisms. However, Retsema and coworkers (35), Nachnani and coworkers (22), and Barry and Fuchs (3) have demonstrated by microdilution MIC testing that the absence of CO2 from the incubation atmosphere when testing azithromycin and erythromycin leads to lower MICs for aerobic as well as anaerobic organisms. Recently, we have developed an agar dilution method using Oxyrase to remove 02 from atmospheric air, and have demonstrated that 94 gram-negative and -positive anaerobes have significantly increased susceptibilities to azithromycin and erythromycin (39).In the current study, the latter findings have been extended by comparing the susceptibilities to erythromycin, azithromycin, clarithromycin, and roxithromycin of 302 anaerobes (170 gram-negative, 132 gram-positive organisms) by the Oxyrase agar dilution technique and agar dilution inside a chamber with CO2. MATERIALS AND METHODSBacterial strains. The organisms-used in the present study (see Table 1) were all clinical strains isolated within 4 years of the stu...

Antibiotic Susceptibility Tests with Fastidious and Nonfastidious Bacterial Reference Strains: Effects of Aerobic versus Hypercapnic Incubation

“…In this context, Rosen blatt and Schoenknecht [201 had noted that under aerobic incubation an initial acidifica tion of Muellcr-Hinton agar was followed by a drift to alkalinity, whereas hypercapnic incu bation enhanced the initial acidification of MHA and reduced the subsequent drift to wards an alkaline pH. Previous investigators [12][13][14][15]20,211 had employed 10% CO:, and this might explain why erythromycin, the quinolones, and the tetracyclines were not antag onized by the lower concentrations of CO, used in this study [211. Our findings permitted the tentative conclusion that incubation of susceptibility tests of p-hemolytic streptococci, pneumococci, and H. influenzae as well as H. parainfluenzae may be incubated under 3% CO, (in a candle jar or in an incubator) or 5% CO, (incubator).…”

“…influenzae and S. pneumoniae that failed to grow optimally under aerobic incubation (with resultant misleadingly large inhibition zones or deceptively low minimal inhibitory concentration (MIC) values); incu bation of repeat tests under hypercapnic conditions, resulting in enhanced growth of the isolates, seemingly alleviated this problem. However, in view of sporadic reports in the lit erature regarding CCL-induced shifts of pH of the media employed with subsequently al tered antibacterial activity of certain antimi crobial drugs, notably aminoglycosides and erythromycin [12][13][14][15], it was decided to exam ine various antimicrobial drugs with three methods of susceptibility testing under three different atmospheric conditions of incuba tion against selected reference strains. …”

Human Necrobacillosis, with Emphasis on Lemierre's Syndrome

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“…The syndrome is too rare to run any randomized, controlled clinical studies. We have most of our information about antibiotic therapy from a few in vitro studies with animal and human strains of F. necrophorum [92][93][94][95][96][97][98][99].…”

Diagnosis and Treatment of Pelvic Inflammatory Disease

“…Fluoroquinolones, including ofloxacin, have generally been found to have limited activity against anaerobes [65][66][67][68]. Azithromycin does provide coverage against a range of anaerobic and aerobic pathogens, including black-pigmented Gram-negative rods [47,[69][70][71][72][73]. A handful of studies demonstrate high rates of clinical cure among women with PID treated with azithromycin [45][46][47][48].…”