Printer Friendly

Effect of in vitro testing parameters on ceftazidime-avibactam minimum inhibitory concentrations.

1. Introduction

Avibactam is a non-[beta]-lactam-[beta]-lactamase inhibitor with activity against class C enzymes, most class A enzymes (including KPCs), and some class D (OXA) carbapenemases [1]. When ceftazidime is combined with avibactam, the spectrum of activity is expanded to include organisms producing these [beta]-lactamases [2-4]. This combination is currently in Phase 3 clinical trials for the treatment of Gram-negative infections.

Susceptibility testing conditions can vary slightly between laboratories and between laboratory scientists. It is important to know if small variations from standard Clinical Laboratory Standards Institute (CLSI) testing guidelines cause any significant changes in testing outcome. These variations can have effects on the MIC when testing [beta]-lactams, including cephalosporins and [beta]-lactam-[beta]-lactamase inhibitor combinations [5-7]. The effect of varying the broth microdilution in vitro testing parameters on the ceftazidime-avibactam MIC of [beta]-lactamase positive and negative Enterobacteriaceae and Pseudomonas aeruginosa isolates was determined and compared with the effects on meropenem and piperacillintazobactam MICs.

2. Materials and Methods

Isolates were obtained from ATCC (Manassas, VA), JMI Laboratories (North Liberty, IA), or the Cerexa, Inc. culture collection. MICs were determined for ceftazidimeavibactam (ceftazidime: USP, avibactam: Forest Laboratories, Inc.), meropenem (USP), and piperacillin-tazobactam (piperacillin: Sigma, tazobactam: USP) by broth microdilution assay according to CLSI guidelines [8, 9]. The effects of the following parameters on the MIC were determined: medium pH 5, 6, and 8 (standard pH 7.2-7.4); incubation with 5% C[O.sub.2] (standard incubation ambient air); the addition of 2.5% laked horse blood (LHB) to cation-adjusted Mueller Hinton broth (CAMHB) and testing in Haemophilus Test Medium (HTM) (standard medium CAMHB); testing in non-cation-adjusted media (MHB without added cations containing 2.9-5.9 mg/L calcium and 3.2-5.2 mg/L magnesium) and testing in 50 mg/L calcium in MHB (standard cation concentration 20-25 mg/L calcium and 10-12.5 mg/L magnesium); and inoculum densities of 5 x [10.sup.4] CFU/mL and 5 x [10.sup.6] CFU/mL (standard density 5 x [10.sup.5] CFU/mL). MIC tests were performed in 2 separate experiments. Data tables list the highest MIC value recorded. Human serum and human serum albumin were previously shown to have no effect on ceftazidime-avibactam MICs [10].

3. Results and Discussion

3.1. Media pH. In medium with acidic pH (pH 5 or 6), MICs for ceftazidime-avibactam were increased by 4- to 16-fold for 6 of 9 Enterobacteriaceae isolates and were decreased by 4-fold for 2 of 4 P. aeruginosa isolates (Table 1). Acidic media caused increased piperacillin-tazobactam MICs against 4 isolates and decreased MICs against 3 isolates by >4-fold (Table 2). Four isolates had 4- to >32-fold decreased piperacillin-tazobactam MICs when tested using medium at pH 8. Acidic media caused increased meropenem MICs against 4 isolates and decreased meropenem MICs against 3 isolates by 4- to 8-fold (Table 3). MIC increases were observed at pH 5 against the reference strain E. coli ATCC 25922 ([beta]-lactamase negative). Thus, the pH effect on MIC appears to be related to the intrinsic antibacterial activity of the [beta]-lactams, rather than having an effect on the inhibition of [beta]-lactamases.

3.2. Incubation Conditions. The ceftazidime-avibactam MIC for one E. coli isolate was increased by 4-fold when incubated in 5% C[O.sub.2] (Table 1). Incubation in 5% C[O.sub.2] caused the piperacillin-tazobactam MICs against 2 P. aeruginosa isolates to change by [greater than or equal to] 4-fold (Table 2). The meropenem MICs for all isolates were unaffected (Table 3).

3.3. Media Composition. Testing in HTM had no effect on MICs with any drug except for one isolate that had 4-fold increased ceftazidime-avibactam MIC (Table 1). Addition of 2.5% LHB had no effect on ceftazidime-avibactam MICs (Table 1), caused a 4-fold decreased piperacillin-tazobactam MIC against one isolate (Table 2), and caused 8-fold and > 32-fold increased meropenem MICs for 2 isolates (Table 3).

3.4. Calcium Content. Ceftazidime-avibactam and piperacillin-tazobactam MICs were unaffected by changes in calcium concentration of media (Tables 1 and 2). In medium with trace calcium (MHB), MICs of meropenem decreased by [greater than or equal to] 4-fold against one isolate and increased by > 4-fold against another (Table 3).

3.5. Inoculum. Three isolates had ceftazidime-avibactam MICs that were increased by 4- to 8-fold when the inoculum was increased 10-fold (5 x [10.sup.6] CFU/mL) (Table 1). Increased inoculum raised the MICs 4- to 16-fold for piperacillintazobactam against 3 isolates, and lower inoculum (5 x [10.sup.4] CFU/mL) resulted in a 4-fold decreased MIC for one isolate (Table 2). High inoculum density caused meropenem MICs to increase by 4- to [greater than or equal to] 64-fold for 7 isolates, while low inoculum density caused meropenem MICs to decrease by 4- to 8-fold for 2 isolates (Table 3).

4. Conclusions

In summary, most of the variations in broth microdilution parameters had little effect on ceftazidime-avibactam MICs. The two parameters that did have an effect were acidic media (6 isolates) and high inoculum density (3 isolates). Changes in MIC for these conditions ranged from 4- to 16-fold. Other changes to testing parameters only affected ceftazidime-avibactam MICs for a single isolate by only 4-fold in each instance. It is possible that these individual cases are due to random variation rather than reproducible effects on MIC. Piperacillin-tazobactam and meropenem MICs were also affected by changes in pH and inoculum density indicating that these effects are not unique to the ceftazidime-avibactam combination. The magnitude of the changes in piperacillintazobactam MICs was more difficult to assess due to the high MICs of most isolates under standard testing conditions.

The inoculum effects noted in this study with ceftazidime-avibactam MICs are not surprising given that inoculum effects have been previously documented with ceftazidime MIC testing, particularly against [beta]-lactamase producing strains [5-7]. An inoculum effect on other cephalosporin and carbapenem MICs has also been previously documented; several studies have shown that MICs for piperacillintazobactam, and to a lesser extent meropenem, are increased with higher inocula of [beta]-lactamase producing and nonproducing isolates [6, 7, 11-13].

These findings indicate that slight variations in testing parameters during routine MIC testing will likely have no significant effect on ceftazidime-avibactam MIC values; however, special attention should be paid to closely follow CLSI guidelines with respect to the pH of the media and the inoculum used. Since most laboratories use commercially prepared media, pH should not be an issue due to the buffering capacity of these media. Additionally, CLSI recommends validating inoculum density on a weekly basis as part of standard quality control so any variations in inoculum should be easily identified [8]. Therefore, if CLSI guidelines and quality control recommendations are followed, there should not be any issues with routine ceftazidime-avibactam MIC testing.

http://dx.doi.org/10.1155/2015/489547

Disclosure

The authors were employees of Cerexa at the time the study was conducted.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This research was funded by Forest Laboratories LLC, a wholly owned indirect subsidiary of Actavis plc, and Cerexa, Inc., a wholly owned subsidiary of Forest Laboratories. Cerexa, Inc. was involved in the design, collection, analysis, and interpretation of data and the decision to present these results.

References

[1] P. Lagace-Wiens, A. Walkty, and J. A. Karlowsky, "Ceftazidimeavibactam: an evidence-based review of its pharmacology and potential use in the treatment of Gram-negative bacterial infections," Core Evidence, vol. 9, pp. 13-25, 2014.

[2] M. Castanheira, S. E. Farrell, K. M. Krause, R. N. Jones, and H. S. Sader, "Contemporary diversity of [beta]-lactamases among enterobacteriaceae in the nine U.S. census regions and ceftazidime-avibactam activity tested against isolates producing the most prevalent [beta]-lactamase groups," Antimicrobial Agents and Chemotherapy, vol. 58, no. 2, pp. 833-838, 2014.

[3] R. K. Flamm, G. G. Stone, H. S. Sader, R. N. Jones, and W. W. Nichols, "Avibactam reverts the ceftazidime MIC90 of European Gram-negative bacterial clinical isolates to the epidemiological cut-off value," Journal of Chemotherapy, vol. 26, no. 6, pp. 333-336, 2014.

[4] H. S. Sader, M. Castanheira, R. K. Flamm, D. J. Farrell, and R. N. Jones, "Antimicrobial activity of ceftazidime-avibactam against gram-negative organisms collected from U.S. medical centers in 2012," Antimicrobial Agents and Chemotherapy, vol. 58, no. 3, pp. 1684-1692, 2014.

[5] C.-I. Kang, M. K. Cha, S. H. Kim et al., "Extended-spectrum cephalosporins and the inoculum effect in tests with CTX-M-type extended-spectrum [beta]-lactamase-producing Escherichia coli: potential clinical implications of the revised CLSI interpretive criteria," International Journal of Antimicrobial Agents, vol. 43, no. 5, pp. 456-459, 2014.

[6] B. Segatore, D. Setacci, M. Perilli et al., "Antimicrobial susceptibility of clinical isolates of Enterobacteriaceae producing complex [beta]-lactamase patterns including extended-spectrum enzymes," International Journal of Antimicrobial Agents, vol. 23, no. 5, pp. 480-486, 2004.

[7] K. S. Thomson and E. S. Moland, "Cefepime, piperacillin-tazobactam, and the inoculum effect in tests with extended-spectrum beta-lactamase-producing Enterobacteriaceae," Antimicrobial Agents and Chemotherapy, vol. 45, no. 12, pp. 3548-3554, 2001.

[8] CLSI, "Performance standards for antimicrobial susceptibility testing; twenty-third informational supplement," CLSI Document M100-S23, Clinical and Laboratory Standards Institute, Wayne, Pa, USA, 2013.

[9] CLSI, Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard, CLSI Document M07-A9, Clinical and Laboratory Standards Institute, Wayne, Pa, USA, 9th edition, 2012.

[10] T. R. Keepers, M. Gomez, C. Celeri, W. W. Nichols, and K. M. Krause, "Bactericidal activity, absence of serum effect, and time-kill kinetics of ceftazidime-avibactam against [beta]-lactamase-producing enterobacteriaceae and Pseudomonas aeruginosa," Antimicrobial Agents and Chemotherapy, vol. 58, no. 9, pp. 5297-5305, 2014.

[11] Y. Harada, Y. Morinaga, N. Kaku et al., "In vitro and in vivo activities of piperacillin-tazobactam and meropenem at different inoculum sizes of ESBL-producing Klebsiella pneumoniae," Clinical Microbiology and Infection, vol. 20, no. 11, pp. O831-O839, 2014.

[12] L. Lopez-Cerero, E. Picon, C. Morillo et al., "Comparative assessment of inoculum effects on the antimicrobial activity of amoxycillin-clavulanate and piperacillin-tazobactam with extended-spectrum [beta]-lactamase-producing and extended-spectrum [beta]-lactamase-non-producing Escherichia coli isolates," Clinical Microbiology and Infection, vol. 16, no. 2, pp. 132-136, 2010.

[13] V. H. Tam, K. R. Ledesma, K. T. Chang, T. Y. Wang, and J. P. Quinn, "Killing of Escherichia coli by [beta]-lactams at different inocula," Diagnostic Microbiology and Infectious Disease, vol. 64, no. 2, pp. 166-171, 2009.

Tiffany R. Keepers, Marcela Gomez, Donald Biek, Ian Critchley, and Kevin M. Krause

Cerexa, Inc., Oakland, CA 94612, USA

Correspondence should be addressed to Tiffany R. Keepers; tkeepers@gmail.com

Received 13 February 2015; Accepted 16 April 2015

Academic Editor: Wladimir Sougakoff
Table 1: Ceftazidime-avibactam MICs with variable
testing parameters.

Ceftazidime-avibactam MIC ([micro]g/mL)

Organism          Isolate #             Genotype             Standard

E. coli          ATCC 25922                WT                  0.5
E. coli            CML2255       CTX-M-15, TEM-1, OXA-1       0.125
E. coli            CML1468                 KPC                 0.5
K. pneumoniae    ATCC 700603             SHV-18                 1
K. pneumoniae      CML138               CTX-M-15                1
K. pneumoniae      CML128         KPC-2, SHV-27, TEM-1          1
K. pneumoniae      CML148      [DELTA]KPC-2, SHV-27, TEM-1     0.25
K. pneumoniae      CML2280       CTX-M-15, TEM-1, OXA-1         4
K. oxytoca         CML2251               TEM-129                1
P. aeruginosa    ATCC 27853                WT                   2
P. aeruginosa      CML1039                AmpC                  8
P. aeruginosa      CML1040            AmpC, TEM-24              2
P. aeruginosa      CML1047                AmpC                  8

                       Ceftazidime-avibactam MIC ([micro]g/mL)

                 5 x [10.sup.4]   5 x [10.sup.6]
Organism             CFU/mL           CFU/mL       pH 5   pH 6   pH 8

E. coli               0.25             0.5          2#     1     0.25
E. coli               0.06             0.5#        0.5#   0.25   0.25
E. coli               0.5               4#          2#     1      1
K. pneumoniae          1                2           4#     4#     1
K. pneumoniae          1                1           1      2     0.5
K. pneumoniae          1                2           4#     4#     1
K. pneumoniae         0.25             0.5          4#     1#    0.5
K. pneumoniae          2                4           2      4      2
K. oxytoca             1                1           1      2      1
P. aeruginosa          2                4           2      2      2
P. aeruginosa          4                16          2#     8      4
P. aeruginosa          2                4           4      4      2
P. aeruginosa          4               >16#         2#     8      8

                        Ceftazidime-avibactam MIC ([micro]g/mL)

                 CAMHB +            Trace    50 mg/L
Organism         2.5% LHB   HTM    calcium   calcium   C[O.sub.2]

E. coli            0.5      0.5     0.25       0.5        0.25
E. coli           0.125     0.25    0.25      0.25        0.5#
E. coli             1        1       0.5       0.5        0.5
K. pneumoniae       1        1        1         1          1
K. pneumoniae       1        1        1         1          1
K. pneumoniae      0.5       4#       1         1          2
K. pneumoniae     0.125     0.25    0.25      0.25        0.25
K. pneumoniae       2        2        4         2          2
K. oxytoca          1        1        1         1          2
P. aeruginosa       2        2        2         2          2
P. aeruginosa       8        8        8         4          16
P. aeruginosa       2        2        2         4          4
P. aeruginosa       8        4        4         8          8

MICs obtained under standard CLSI conditions are indicated
by the "standard" column header. Parameters that were varied
include inoculum density (5 x [10.sup.4] CFU-mL and 5 x
[10.sup.6] CFU-mL), pH of medium (pH 5,6, and 8), medium
supplements (2.5% laked horse blood (LHB) and HTM), cation
content (trace amounts of calcium and 50 mg-L calcium), and
incubation with 5% C[O.sub.2]. Bolded values indicate a
change in MIC of [greater than or equal to] 4-fold.

Table 2: Piperacillin-tazobactam MICs with variable
testing parameters.

                Piperacillin-tazobactam MIC ([micro]g/mL)

Organism          Isolate #             Genotype             Standard

E. coli          ATCC 25922                WT                   2
E. coli            CML2255       CTX-M-15, TEM-1, OXA-1         32
E. coli            CML1468                 KPC                 >512
K. pneumoniae    ATCC 700603             SHV-18                 16
K. pneumoniae      CML138               CTX-M-15               >512
K. pneumoniae      CML128         KPC-2, SHV-27, TEM-1         >512
K. pneumoniae      CML148      [DELTA]KPC-2, SHV-27, TEM-1     256
K. pneumoniae      CML2280       CTX-M-15, TEM-1, OXA-1        >512
K. oxytoca         CML2251               TEM-129               >512
P. aeruginosa    ATCC 27853                WT                   4
P. aeruginosa      CML1039                AmpC                 512
P. aeruginosa      CML1040            AmpC, TEM-24              64
P. aeruginosa      CML1047                AmpC                 >512

                    Piperacillin-tazobactam MIC ([micro]g/mL)

                 5 x [10.sup.4]   5 x [10.sup.6]
Organism             CFU/mL           CFU/mL       pH 5   pH 6   pH 8

E. coli                2                4           8#     4      2
E. coli                16              512#         64    128#    16
E. coli               512              >512        256    >512   512
K. pneumoniae          8                16         64#    64#     8
K. pneumoniae         >512             >512        >512   >512   16#
K. pneumoniae         >512             >512        >512   >512   >512
K. pneumoniae         64#             >512#        256    256    16#
K. pneumoniae         256              >512        >512   >512   32#
K. oxytoca            >512             >512        512    >512   256#
P. aeruginosa          4                8           8      4      4
P. aeruginosa         512              >512        128#   512    >512
P. aeruginosa          64              256#        256#   256#    32
P. aeruginosa         512              >512        128#   512    >512

                 Piperacillin-tazobactam MIC ([micro]g/mL)

                 CAMHB +            Trace    50 mg/L
Organism         2.5% LHB   HTM    calcium   calcium   C[O.sub.2]

E. coli             2        2        2         2          4
E. coli             16       64      32        32          64
E. coli            512      >512    >512      >512        >512
K. pneumoniae       16       16      16        16          8
K. pneumoniae      >512     >512    >512      >512        >512
K. pneumoniae      >512     >512    >512      >512        >512
K. pneumoniae      64#      256      128       128        256
K. pneumoniae      >512     >512    >512      >512        >512
K. oxytoca         >512     >512    >512      >512        >512
P. aeruginosa       2        2        2         4          4
P. aeruginosa      512      512      512       512        512
P. aeruginosa       64       64      64        128        256#
P. aeruginosa      512      512     >512      >512        256#

MICs obtained under standard CLSI conditions are indicated
by the "standard" column header. Parameters that were varied
include inoculum density (5 x [10.sup.4] CFU-mL and 5 x
[10.sup.6] CFU-mL), pH of medium (pH 5,6, and 8), medium
supplements (2.5% laked horse blood (LHB) and HTM), cation
content (trace amounts of calcium and 50 mg-L calcium), and
incubation with 5% C[O.sub.2]. Bolded values indicate a change in
MIC of [greater than or equal to] 4-fold.

Note: Change in MIC of [greater than or equal to] 4-fold are
indicated with #.

Table 3: Meropenem MICs with variable testing parameters.

                   Meropenem MIC ([micro]g/mL)

Organism           Isolate #             Genotype

E. coli           ATCC 25922                WT
E. coli             CML2255       CTX-M-15, TEM-1, OXA-1

E. coli             CML1468                 KPC
K. pneumoniae     ATCC 700603             SHV-18
K. pneumoniae       CML138               CTX-M-15

K. pneumoniae       CML128         KPC-2, SHV-27, TEM-1
K. pneumoniae       CML148      [DELTA]KPC-2, SHV-27, TEM-1
K. pneumoniae       CML2280       CTX-M-15, TEM-1, OXA-1
K. oxytoca          CML2251               TEM-129

P. aeruginosa     ATCC 27853                WT
P. aeruginosa       CML1039                AmpC
P. aeruginosa       CML1040            AmpC, TEM-24
P. aeruginosa       CML1047                AmpC

                       Meropenem MIC ([micro]g/mL)

                                                 5 x [10.sup.4]
Organism           Isolate #       Standard          CFU/mL

E. coli           ATCC 25922        0.016            0.016
E. coli             CML2255     [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
E. coli             CML1468           4                1
K. pneumoniae     ATCC 700603        0.03            0.016
K. pneumoniae       CML138      [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
K. pneumoniae       CML128           >64               32
K. pneumoniae       CML148           0.25             0.25
K. pneumoniae       CML2280          0.06             0.03
K. oxytoca          CML2251     [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
P. aeruginosa     ATCC 27853         0.5              0.5
P. aeruginosa       CML1039           64               32
P. aeruginosa       CML1040           4                4
P. aeruginosa       CML1047           16               16

                     Meropenem MIC ([micro]g/mL)

                                5 x [10.sup.4]
Organism           Isolate #        CFU/mL        pH 5

E. coli           ATCC 25922        0.125#       0.125#
E. coli             CML2255         0.25#        0.125

E. coli             CML1468          >8#           1#
K. pneumoniae     ATCC 700603        0.06         0.06
K. pneumoniae       CML138            2#         0.125

K. pneumoniae       CML128           >64           64
K. pneumoniae       CML148            4#          0.25
K. pneumoniae       CML2280          0.5#        0.125
K. oxytoca          CML2251           4#         0.125

P. aeruginosa     ATCC 27853         0.5          0.5
P. aeruginosa       CML1039           64           8#
P. aeruginosa       CML1040           8            4
P. aeruginosa       CML1047           16           4#

Meropenem MIC ([micro]g/mL)

Organism           Isolate #         pH 6             pH 8

E. coli           ATCC 25922         0.06#            0.03
E. coli             CML2255     [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
E. coli             CML1468           4                4
K. pneumoniae     ATCC 700603        0.03             0.06
K. pneumoniae       CML138          0.125        [less than or
                                                 equal to] 0.06
K. pneumoniae       CML128           >64              >64
K. pneumoniae       CML148           0.25             0.5
K. pneumoniae       CML2280          0.06             0.06
K. oxytoca          CML2251     [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
P. aeruginosa     ATCC 27853         0.25             0.5
P. aeruginosa       CML1039           32               64
P. aeruginosa       CML1040           4                8
P. aeruginosa       CML1047           4#               32

                     Meropenem MIC ([micro]g/mL)

                                   CAMHB +
Organism           Isolate #       2.5% LHB           HTM

E. coli           ATCC 25922         0.03             0.03
E. coli             CML2255     [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
E. coli             CML1468           4                8
K. pneumoniae     ATCC 700603       0.25#             0.03
K. pneumoniae       CML138      [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
K. pneumoniae       CML128            64              >64
K. pneumoniae       CML148           0.25             0.25
K. pneumoniae       CML2280          0.06             0.06
K. oxytoca          CML2251           2#         [less than or
                                                 equal to] 0.06
P. aeruginosa     ATCC 27853         0.5              0.5
P. aeruginosa       CML1039           32               32
P. aeruginosa       CML1040           4                4
P. aeruginosa       CML1047           16               16

                           Meropenem MIC ([micro]g/mL)

                                                    50 mg/L
Organism           Isolate #    Trace calcium       calcium

E. coli           ATCC 25922        0.016             0.03
E. coli             CML2255     [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
E. coli             CML1468           2                4
K. pneumoniae     ATCC 700603       0.016            0.016
K. pneumoniae       CML138      [less than or    [less than or
                                equal to] 0.06   equal to] 0.06
K. pneumoniae       CML128           32#               64
K. pneumoniae       CML148          0.125             0.25
K. pneumoniae       CML2280          0.06            0.125
K. oxytoca          CML2251         0.25#             0.5#

P. aeruginosa     ATCC 27853         0.5              0.25
P. aeruginosa       CML1039           32               32
P. aeruginosa       CML1040           4                8
P. aeruginosa       CML1047           8                16

                  Meropenem MIC ([micro]g/mL)

Organism           Isolate #      C[O.sub.2]

E. coli           ATCC 25922         0.03
E. coli             CML2255     [less than or
                                equal to] 0.06
E. coli             CML1468           4
K. pneumoniae     ATCC 700603        0.03
K. pneumoniae       CML138      [less than or
                                equal to] 0.06
K. pneumoniae       CML128            64
K. pneumoniae       CML148          0.125
K. pneumoniae       CML2280          0.06
K. oxytoca          CML2251     [less than or
                                equal to] 0.06
P. aeruginosa     ATCC 27853         0.25
P. aeruginosa       CML1039           32
P. aeruginosa       CML1040           8
P. aeruginosa       CML1047           8

MICs obtained under standard CLSI conditions are indicated
by the "standard" column header. Parameters that were varied
include inoculum density (5 x [10.sup.4] CFU-mL and 5 x
[10.sup.6] CFU-mL), pH of medium (pH 5,6, and 8), medium
supplements (2.5% laked horse blood (LHB) and HTM), cation
content (trace amounts of calcium and 50 mg-L calcium), and
incubation with 5% C[O.sub.2]. Bolded values indicate a
change in MIC of [greater than or equal to] 4-fold.

Note: Change in MIC of [greater than or equal to] 4-fold are
indicated with #.
COPYRIGHT 2015 Hindawi Limited
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2015 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Research Article
Author:Keepers, Tiffany R.; Gomez, Marcela; Biek, Donald; Critchley, Ian; Krause, Kevin M.
Publication:International Scholarly Research Notices
Date:Jan 1, 2015
Words:3477
Previous Article:IMPATT diodes based on (111), (100), and (110) oriented GaAs: a comparative study to search the best orientation for millimeter-wave atmospheric...
Next Article:Improved iterative decoding of network-channel codes for multiple-access relay channel.
Topics:

Terms of use | Privacy policy | Copyright © 2022 Farlex, Inc. | Feedback | For webmasters |