Journal of Antimicrobial Chemotherapy (1999) 44, 819-822
© 1999 The British Society for Antimicrobial Chemotherapy
Brief report |
Tentative minimum inhibitory concentration and zone diameter breakpoints for moxifloxacin using BSAC criteria
Department of Antimicrobial Chemotherapy, City Hospital NHS Trust, Dudley Road, Birmingham B18 7QH, UK
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Abstract |
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Tentative MIC and zone diameter breakpoints were determined for moxifloxacin using BSAC criteria. An MIC breakpoint of
1 mg/L, denoting sensitivity, is suggested for
Enterobacteriaceae, staphylococci, haemophili, moraxellae, pneumococci and enterococci. For
pseudomonads high and low breakpoints of 4 mg/L and 1 mg/L are suggested to allow for an
intermediate category of sensitivity. A 1 µg moxifloxacin disc content is suggested for
testing all of the organisms previously mentioned, except pseudomonads, for which a 5 µg
disc is needed to discriminate between the intermediate and sensitive populations. Corresponding
zone diameter breakpoints for a 1 µg disc are 
20 mm for Enterobacteriaceae and
staphylococci, 18 mm for the respiratory pathogens (Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis) and 15 mm for enterococci. For
Pseudomonas aeruginosa with a 5 µg disc, three bands are suggested for
interpretation, that of 25 mm (sensitive), 18-24 mm (intermediate) and 17 mm
(resistant). |
Introduction |
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TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
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Moxifloxacin is a new 8-methoxyquinolone with activity against a wide range of organisms including respiratory pathogens and in particular Streptococcus pneumoniae.1 This study was designed to determine MIC and zone diameter breakpoints for moxifloxacin using methodology and criteria recommended by the BSAC.2
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
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MICs and disc testing
MICs were determined and disc testing was performed following the recommendations of the BSAC.2 Briefly, Iso-Sensitest agar (Oxoid, Basingstoke, UK) was used as basal medium. For the growth of fastidious organisms the basal medium was supplemented with 5% whole horse blood (E&O Laboratories, Bonnybridge, UK) or 5% whole horse blood and 20 mg/L ß-nicotinamide adenine dinucleotide (NAD) (Sigma Diagnostics, Poole, UK) where necessary. For disc testing, medium was poured into 90 mm Petri dishes to give a depth of 4 mm. The inoculum used for MICs was 104 cfu/spot and for disc testing, that which would yield semi-confluent growth. Plates were incubated at 35–37°C for 18–20 h in air except for fastidious organisms (including pneumococci) for which the atmosphere was enriched with 4–6% CO2. The disc contents of moxifloxacin used were 1 µg, 2 µg and 5 µg. The 5 µg discs were obtained from Oxoid and the 1 µg and 2 µg discs were prepared by City Hospital. Sterile 6 mm blotting paper discs (Whatman, Maidstone, UK) were impregnated with solutions of moxifloxacin, the concentrations of which were derived from the following equation:
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where DC is the disc content (µg), VA is the volume absorbed by the disc (µL) and AC is the concentration of antibiotic which when the VA is applied to the disc will give a disc content DC.
Bacteria
A total of 781 strains were included in the study, comprising 100 S. pneumoniae, 50 Haemophilus influenzae, 50 Moraxella catarrhalis, 130 staphylococci (including Staphylococcus aureus, coagulase-negative staphylococci and Staphylococcus saprophyticus), 50 enterococci (including Enterococcus faecalis and Enterococcus faecium), 320 species of Enterobacteriaceae, 50 Pseudomonas aeruginosa, 20 Acinetobacter spp. and 11 NCTC/ATCC control strains.
Antimicrobial agent
Moxifloxacin was provided by Bayer AG (Wuppertal, Germany).
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Results and discussion |
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TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
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MIC results for the control strains included in the study are shown in the Table.
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Establishing an MIC breakpoint
To assist interpretation of sensitivity an in-vitro breakpoint MIC has to be established. This is determined by using pharmacokinetic data, MIC distribution data for the organisms to be targeted by the drug or a combination of both.
Applying the BSAC formula: 2
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(where Cmax is the peak blood concentration, f is a factor allowing for protein binding, t is a factor allowing for half life, e is the factor by which the Cmax should exceed the MIC and s is the shift needed to allow for optimum reproducibility) to plasma concentrations obtained after a single 400 mg oral or iv dose of moxifloxacin (5.0 mg/L and 5.1 mg/L, respectively3) an in-vitro breakpoint concentration of 2.5 mg/L was calculated.
Strains (86.2%, excluding pseudomonads) considered to be part of the ‘wild-sensitive
population', with no putative mechanism of resistance to quinolones, were inhibited by 1
mg/L moxifloxacin (data not shown). From these data an MIC breakpoint of 1 mg/L is
suggested for all of the species studied, with the exception of pseudomonads. An MIC breakpoint
of 1 mg/L for respiratory pathogens is endorsed by Grimm et al.4 The results of clinical trials undertaken by Krasemann et al.5 suggest that 89% to 97% of causative pathogens can be
expected to be eradicated following treatment of respiratory infections with 400 mg od
moxifloxacin, when causative strains have MICs 2 mg/L.5
Pseudomonas aeruginosa strains were less susceptible to moxifloxacin, with 42% and 72% of strains being inhibited by 1 mg/L and 2 mg/L, respectively (data not shown) and therefore two MIC breakpoints are suggested: 4 mg/L (high) and 1 mg/L (low), allowing for a category of intermediate sensitivity.
A summary of the suggested MIC breakpoints for each of the genera studied is shown in the Table.
Establishing a zone diameter breakpoint
Zones of inhibition for all genera except P. aeruginosa, were acceptable for a 1 µg disc (range 18–35 mm), but unacceptably large for 2 µg and 5 µg disc (zone diameters ranging from 25 mm to 40 mm and 27 mm to 43 mm for 2 µg and 5 µg discs, respectively). For pseudomonads, acceptable zone ranges were only obtained with the 5 µg disc (range 18–31 mm). For the 1 µg and 2 µg discs, relatively small zones were produced which did not discriminate between strains with sensitive and intermediate susceptibility (zone diameter ranges 16–26 mm and 9–18 mm for 2 µg and 1 µg discs, respectively).
The Figure illustrates the data for all of the genera tested except Pseudomonas, Serratia and Acinetobacter spp. Using a zone diameter
breakpoint of 20 mm to denote sensitivity of Enterobacteriaceae and staphylococci, no false
sensitivity was observed and false-resistant rates of 10.6% and 5% were seen for
Enterobacteriaceae and staphylococci, respectively. The high level of false resistance observed
for
the Enterobacteriaceae was mainly amongst strains of Escherichia coli, Klebsiella
spp. and Proteus mirabilis. MICs of moxifloxacin for E. coli and Klebsiella
spp. were higher than those of the ‘wild-sensitive' population for
these genera (0.25 mg/L to 1 mg/L), which may indicate the presence of a mechanism of
resistance of clinical importance. However, for the seven strains of P. mirabilis with
MICs of 0.25 mg/L to 0.5 mg/L and correspondingly small zones (14 mm to 19 mm) there is no
obvious explanation for false resistance.
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None of the Acinetobacter or Serratia spp. was designated as falsely sensitive; however, false resistance was seen for six serratia with MICs of moxifloxacin between 0.5 mg/L and 1 mg/L and two acinetobacter with MICs of 0.5 mg/L and 0.015 mg/L. For these genera it may be prudent to follow the recommendations of the BSAC and to determine sensitivity using an MIC method.2
For the respiratory pathogens studied, a zone diameter breakpoint of 18 mm was chosen. No false sensitivity was observed for any of the genera tested and false resistance was only observed amongst three strains of pneumococci (1.5%) with MICs of moxifloxacin of 0.25 mg/L.
For enterococci, a zone diameter breakpoint of 15 mm yielded no false resistance, but two strains of E. faecium with MICs of 2 mg/L and 8 mg/L were interpreted as falsely sensitive.
With regard to P. aeruginosa, analysis was undertaken using the data derived from
the 5 µg disc (data not shown). In this instance, an intermediate category of sensitivity was
defined (Table). Of the strains tested, amongst the resistant population (n = 13) (MICs 8 mg/L) no false sensitivity was observed. Of the strains
with MICs between 2 mg/L and 4 mg/L (n = 16) seven were interpreted as
sensitive and amongst the sensitive population (MICs 1 mg/L) (n = 21), all
were correctly interpreted as sensitive.
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Notes |
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* Corresponding author. Tel: +44-121-554-3801; Fax: +44-121-551-7763.
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References |
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TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
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1 . Dalhoff, A., Petersen, U.& Endermann, R. (1996). In vitro activity of BAY 12-8039, a new 8-methoxyquinolone. Chemotherapy 42, 410–25.[Web of Science][Medline]
2
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A Guide to Sensitivity Testing. (1991). Journal of Antimicrobial Chemotherapy 27, Suppl. D, 1–50.
3 . Wise, R., Andrews, J. M.& Hartman, G. (1998). The pharmacokinetics and penetration of moxifloxacin into a cantharidine induced inflammatory exudate following oral and intravenous administration. In Program and Abstracts of the Thirty-Eighth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Diego, CA, 1998. Abstract A27, p. 8. American Society for Microbiology, Washington, DC.
4 . Grimm, H. (1999). Moxifloxacin: tentative interpretative criteria for the disc susceptibility testing using the German DIN 58940 method. In Abstracts of the Ninth European Congress of Clinical Microbiology and Infectious Diseases, Berlin, Germany, 1999. Abstract P220, p. 143. Clinical Microbiology and Infection 5, Suppl. 3.
5 . Krasemann, C., Meyer, J. M.& Springsklee, M. (1999). Suggested breakpoints for moxifloxacin. In Abstracts of the Ninth European Congress of Clinical Microbiology and Infectious Diseases, Berlin, Germany, 1999. Abstract P206, p. 139. Clinical Microbiology and Infection 5, Suppl. 3.
Received 22 February 1999; returned 27 May 1999; revised 16 June 1999; accepted 27 July 1999
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