TOR (Thermo Fisher Oxoid/Remel chromogenic plate) detected GBS with 98% sensitivity after 18 hours of incubation, which was higher than SEL (Bio-Rad GBS Select chromogenic plate) at 95% and SBA (blood agar plate with gentamicin disc) at 87.5%. TOR also had a higher specificity of 84% compared to 97% for SEL and 95% for SBA. TOR allowed for results to be reported one day earlier than SEL and SBA. TOR showed less breakthrough growth of other organisms and better inhibited non-GBS beta-hemolytic streptococci compared to SEL and SBA.

1. . .
Comparison of Two Chromogenic Media and an
Enhanced Blood Plate Method for the Purpose of GBS
Detection from Broth Enriched Vaginal/Rectal Samples
P. Kornherr, M. Michelin, N. Rundle, K.Delcorde
Department of Microbiology, Dynacare Ottawa, Canada
Table 2: Sensitivities and Specificities at 18 hrs. incubation
Figure 5:Clockwise from top right:
GBS grown on SBA, TOR and SEL.
Table 1 : 1) Recovery of GBS from 300 patient samples. 27% positivity
,
Introduction Method
Method
Results
Conclusion
Acknowledgements
Results
Group B Streptococcus (GBS) is the most frequent cause of
systemic infections in neonates less than one week old. Since
laboratories have implemented the CDC guidelines for GBS
detection 10 years ago, there has been a steady decline in mortality
due to this organism. Correct laboratory processing of specimens
for GBS screening is paramount to the success of universal
screening. Most cases of mortality now occur in infants born to
women who had been screened negative. An optimized method for
each laboratory is necessary and CDC has allowed for some
options as per the 2010 guidelines. Pigmented agars and broths as
well as nucleic acid amplification tests and chromogenic agars
have successfully since debuted, each with their advantages and
disadvantages. Chromogenic agars, in particular, are become more
common as they are highly cost-effective, sensitive and allow for
much easier detection of GBS than at-best modest hemolysis from
a blood agar plate. Furthermore blood medias , technologist
experience and agglutination cross-reactions lend themselves to a
much needed extra tool for GBS detection –that of a simple
specific coloured colony. We investigated the performance of a
GBS chromogenic plate from Thermo Fisher Oxoid/Remel that
required only 18 hrs. incubation. In this study it will be referred to
as TOR. The existing and proven other chromogenic plate from
Bio-Rad GBS Select (SEL) was compared with TOR in this study
as well as the standard blood plate with an enhanced gentamicin
disc applied (SBA). The latter modification allows for GBS
discrimination from Enterococcus species due to differences in
gentamicin susceptibility. The enhancement is particularly
effective when dealing with non-hemolytic GBS differentiation.
All of these chromogenic media manufacturers allow for the
option of inoculating their plates directly. However CDC requires
broth enrichment or the detection of GBS may be sub-optimal.
Through our own studies and published data, most of these plates
may reach only 60-70% sensitivity of GBS detection by direct
inoculation only without broth enrichment. It is allowable by CDC
to add a plate prior to broth inoculation as an adjunct to broth
subbing and this has shown to increase the yield of GBS by 10%.
This is usually attributable to certain Enterococcus sp.
competitively inhibiting GBS growth in broth . CDC also allows
reading of plates according to manufacturer so using TOR would
reduce TAT by one day when compared to routine SBA and SEL
requirements.
1)300 pre-natal vaginal/rectal samples received for GBS detection
were inoculated into a Thermo-Fisher Oxoid LIM broth and
incubated 18-24 hrs. To each BAP 2nd quadrant, a 10 µgm.
gentamicin disc was added. All plates were incubated for 18-24
hrs. and read for GBS colonies (BRI - pink; SEL- turquoise;
BAP- typical grey, hemolytic and growth up to gent disc.) . SEL
Figure 1: Sample 231:
Mixture of Group C and
Group B Streptococcus on
SBA
Figure 2: Sample 231:
Mixture of Group C
and B Streptococcus
on SEL
T
Table 5: colony sizes
Sensitivity TOR>SEL>BAP.
Specificity SEL>BAP>TOR.
All typical TOR results can be released 24 hrs earlier than SEL
and BAP.
TAT TOR<SEL=BAP.
Colony size TOR>SEL>BAP.
Breakthrough growth TOR <SEL <BAP.
Beta Hemolytic Streptococci non-group B inhibition :
TOR>SEL>SBA
All grew non-hemolytic GBS well. Non haemolytic GBS,
unlike Enterococcus, shows growth up to gentamicin disc on
BAP. with the enhanced method.
Group A, C, G are inhibited on TOR.
Figure 6: Clockwise from top
right: Enterococcus faecalis
grown on SEL, TOR and SBA
Figure 4 (all): Clockwise from top right for each quadrant/plate : Gr. A
Streptococcus, Group C Streptococcus , Group G Streptococcus and
Group B Streptococcus on L-R SBA, SEL and TOR
Figure 3:Sample 231 :
Group C absent.Group
B Streptococcus only
on TOR
Figure 5: Figure 6: 2
The authors wish to thank the microbiology department at Dynacare, Bio-
Rad and Thermo Fisher Scientific for their support .
Table 3: Phase 2) Relative amounts of breakthrough contaminants
per plate and recovery.
TOR
(18 hrs
incubation)
SEL
(18 hrs and 24
hrs. incubation)
SBA
(18 hrs and 24 hrs.
incubation)
Positive 79 76 70
Negative 221 224 230
Exclusive
Recoveries
6 1 0
False Positives 42 7 12
TOR (%) SEL (%) SBA (%)
Sensitivity 98 95 87.5
Specificity 84 97 95
TOR SEL SBA
Positives 44 41 40
# of plates with
mixed flora
13 35 44
Recovery 100% 93% 91%
Media Colony Size
18 hrs 48 hrs
TOR 0.89 mm 1.3 mm
SEL 0.79 mm .91 mm
SBA 0.83 mm 1.0 mm
(and BAP) were reincubated for additional 24 hrs. as per
manufacturer and/or CDC. Confirmation was via PathoDxtra B
latex agglutination. PYR Catalase, gram, Vitek MS used as
necessary. A positive GBS was defined as agglutination etc. positive
from any one or combination of positive plates. 2) 44 known
positive GBS swabs had their respective broths subbed to each
plate and incubated as above. Recovery and mixed growths were
noted. Swabs were deemed positive by either a pre-broth
inoculated SEL plate or broth subculture to SBA as part of usual
work-up algorithm.
1)80 samples recovered GBS (27%). 21% is the provincial
average. BRI detected 79; SEL:76 ; BAP:70 with respective
sensitivities of 98%, 95%, and 87.5%. 7 of SEL and 8 BAP
samples required reincubation or subculturing to achieve useable
growth for testing. All GBS growths detected at 18-24 hrs.
Specificity was BRI: 84%; SEL:97 %; BAP:95%.
2 ) All 44 pos. recovered on BRI (100%) ; 41 on SEL (93%) and
40 on BAP (91%). BRI 30% mixed; SEL 79% mixed; BAP
100% mixed with breakthrough flora. 6 GBS strains were
deemed non-hemolytic with recovery on all plates.
Results
Non - Hemolytic GBS mixed with Enterococcus faecalis
using a 10 µgm. gentamicin disc to differentiate on SBA .
Note GBS grows up to disc. E. Faecalis shows zone of no
growth around disc.