A COMPARISON OF VARIOUS MEDIA FOR THE GROWTH OF CARBAPENEMASE-PRODUCING ORGANISMS
David J. M. Haldane2,3,4and Carrie Phillips1,2
1Public
Health Agency of Canada, Winnipeg, Manitoba, Canada; 2Provincial Public Health Laboratory Network, Halifax, N.S., Canada;
3Department of Pathology, Dalhousie University and; 4Division of Microbiology, Department of Pathology and Laboratory Medicine, Nova Scotia Health Authority (NSHA), Halifax, N.S., Canada
Introduction and Objective:
Results:
• Many species of carbapenem-producing organisms (CPO) having different
carbapenemases may colonize patients .
Table 2. Sensitivity /Specificity of Selective Media for CPO
Table 3. Growth of isolates of CPOs and non-CPOs on selective
media at 24 h (48h) using different inocula.
• Detection of carriers is useful for control of spread of CPO.
• Sensitive and specific screening media are desirable to minimize
workload.
• Screening media were assessed for sensitivity and specificity of detection
of different CPO.
Methods:
• the growth of CPOs and non-CPOs was compared on the following solid
media: Meropenem (0.2mg/L)-MacConkey (in house); Carba Smart
OXA/Carb in a biplate (bioMerieux); Chromogenic ESBL/CRE media in a
biplate and CRE in a full plate (OXOID); Colorex SuperCarba medium
(Dalynn Biologicals); Sheep blood agar (5% SBA) was used as a nonselective medium.
This table shows the overall sensitivity. Differences amongst media and enzyme
types are shown in Table 3. The sensitivity of detection was highest for the greatest inoculum. The range
is shown for clarity. The growth on the SBA was the reference standard (not shown). The CarbaSmart
OXA detected only the OXA 48 containing strains and did not improve on the detection of isolates with this
enzyme by the CarbaSmart Carb plate.
• the organisms used are listed in Table 1, comprising:
 35 CPOs, including some of which contained additional
lactamases
 45 non-CPOs isolates: 24 with one or more β- lactamases
as AmpC or ESBL and 21 with no genes
detected by PCR
βThe enzyme types and number of strains tested (n) are listed on the left.
The “Log” is the inoculum. Results for inocula of log 2-4 are shown on a
single line if they were the same. The number of strains isolated on each
medium at each dilution is shown. Colony colour was not used. *values
differing between 2 replicates of testing.
such
• a suspension of a culture grown on SBA was serially diluted and four
dilutions were compared
• a 32-pin replicator tool was used to deliver ~8 ul bacterial suspension to
the surface of the agar (Log 4 to Log 1 cfu/spot)
Conclusions:
• growth and color of the colonies was recorded at 18-24 h and 48 h, and
repeated one week later.
Figure 1. The ability of CPOs to grow on the selective media: 24 and 48 h.
• Colony colour did not augment detection of CPO.
• SME containing isolates were detected by CarbaSmart Carb, and
Meropenem MAC but not the Chromogenic ESBL/CRE, or the
SuperCarba.
• The CarbaSmart Carb was the most sensitive and specific medium
overall.
Table 1. Isolates used in the study
• Meropenem MAC was sensitive but lacked specificity.
• As the non CPO challenge set included many isolates with ESBL,
specificity might be higher in routine practice.
Figure 2. The growth of non-CPO.
• At 24 hours the specificity was increased with minimal loss of sensitivity
for CarbaSmart Carb and Chromogenic plates, but was the optimal time for
the SuperCarba.