Of the 5 373
patients, 71% had isolates submitted to a public health laboratory and the antimicrobial
susceptibility testing was performed on 77.1% (2 937).
The plurality of isolates were from Minnesota (46.8% or 1 375),
followed by Georgia (26.1% or 768), New York (16.9% or 496), and
Oregon (10.2% or 300). The perinatal category comprised 24.4% (1 311)
of the total sample with 12.5% (671) early-onset, 7.9% (424) late-onset, and
4.0% (216) maternal patients. The non-pregnant adult category accounted for
73.5% (3 951) of the total.
All 2 937
isolates were susceptible to penicillin, ampicillin, cefotaxime, and
vancomycin. Although no CLSI
breakpoints existed for the first-generation cephalosporins (cephalothin and
cefazolin), all the isolates tested had minimum inhibitory concentrations
(MICs) ≤ 0.5 mcg/mL and were, therefore, considered susceptible.
Similarly, no breakpoints existed for cefuroxime and cefoxitin. All 1 201
Minnesota isolates tested for cefuroxime susceptibility had MICs ≤ 0.12 mcg/mL
and were, therefore, considered susceptible. Of 2 937
isolates tested for cefoxitin susceptibility, 36.7% (1 078) had MICs ≤ 2 mcg/mL; 62.5%
(1 835) had MICs = 4 mcg/mL; 0.7% (22) had
MICs = 8 mcg/mL; and 0.1% (4) had MICs ≥ 16 mcg/mL.
resistance was identified in 12.7% (374), and erythromycin resistance was
identified in 25.6% (752) of isolates tested. During the study period, a
significant increase was observed in the proportion of isolates resistant to
each of these antibiotic
(for clindamycin 10.5% in 1996 versus 15.0% in 2003, X2 for trend = 12.70, P < .001, for erythromycin
15.8% in 1996 versus 32.8% in 2003, X2 for trend = 55.46, P < .001) (see ). Concurrent resistance to both clindamycin and
erythromycin was identified in 12.6% (370) of isolates tested.
Clindamycin and erythromycin resistance among invasive Group B streptococci isolates by year (n = 2 937 isolates).
differences were identified between Minnesota and New York
isolates in clindamycin resistance (13.2% versus 9.3%, P = .03) and
erythromycin resistance (24.5% and 31.4%, P < .01). Georgia isolates were more likely to be
resistant to clindamycin than New York isolates (15.1% and 9.3%,
P = .02), but Georgia isolates were less likely to be resistant
to erythromycin than to New York isolates (26.3% and 31.4%, P = .04) (see ).
Clindamycin and erythromycin resistance among invasive Group B streptococci isolates by state (n = 2 937 isolates).
The proportion of
isolates resistant to clindamycin and erythromycin did not differ significantly
between the perinatal and non-pregnant adult patient groups (see ).
Within the perinatal group, the overall clindamycin resistance was 13.1%, with
no significant changes between the early-onset (14.9%), late-onset (10.3%), and
maternal populations (13.7%). Likewise, no significant differences in
erythromycin resistance levels were identified between the different perinatal
groups (21.9% overall; 21.2% for early-onset, 22.1% for late-onset, and 23.9%
Resistance invasive Group B streptococci isolates among pregnant women and newborns (perinatal) and non-pregnant adults (n = 2 887 isolates).
Serotyping was performed on 64.6% (3 471) of isolates. The largest proportion was
from Georgia (34.7%), followed by Minnesota (28.5%), New York (23.9%), and
Oregon (12.9%). illustrates the serotypes identified during the study period,
with predominant serotype groups being V, Ia, and III. The proportion of
isolates with serotype III was observed to have a significant downward trend (X2 trend = 38.8; P < .001) during the study period from 25% in 1996 to 17%
in 2003; no such trend was observed with the other predominant serotypes.
Distribution of serotypes and resistance to erythromycin and clindamycin among
invasive group B streptococcus isolates.
The most common serotype in the perinatal
population was serotype III (37.0%; 351/948). This was significantly different
than for non-pregnant adults in which serotype V was most common (31.5%; P < .001).
Both susceptibility and serotyping testing were
performed on 2 551 isolates. The largest proportion of these isolates were
from Minnesota (38.9%), followed by Georgia (30.1%), New York (19.4%), and
Oregon (11.8%). Clindamycin resistance was 3.9 times (P < .001) more likely to occur in
serotype V isolates (25.8%) than in other serotypes (8.2%). In addition,
erythromycin resistance was 2.9 times (P < .001) more likely to occur
in serotype V isolates (42.2%) than in other serotypes (20.2%). Subanalysis
within the different disease categories indicated a significant association
among clindamycin or erythromycin resistance and early-onset (P < .001),
late-onset (P < .001), and non-pregnant adult populations (P < .001).
No such association was observed among the maternal population.
hundred fifty-five erythromycin-resistant (47%) isolates were further
characterized. Seventy-eight isolates (22%) had an M phenotype and all of these
isolates contained mef only. Fifty-two percent (184/355) were
constitutively resistant to clindamycin (cMLS). Seventeen were further tested. Ten were positive for ermB and
seven contained ermTR; one isolate
had both mef and ermTR and one isolate had both mef and ermB. Twenty-five percent of isolates had inducible
clindamycin resistance (iMLS). All of the iMLS isolates were tested for
resistance genes; 83 (92%) contained ermTR,
1 isolate contained ermB, and 6 isolates
were negative for ermTR and ermB.
Univariate analysis was conducted comparing
resistance genes to serotypes. Serotype V was associated with a lower
proportion of mef positive isolates
compared to all other serotypes (P < .01). In contrast, serotypes Ia,
Ia/c, and III were associated with a higher proportion of mef-positive isolates compared to all other serotypes (all P values <.01). Serotype V had a higher proportion of isolates with ermTR, 73% of 37 tested
compared with 29% among all other serotypes (P < .01).