Bacterial load

Eleven studies reported on bacterial load.276, 278, 103, 279-285, 90 _{The definition of bacterial load}

differed across studies. The number of positive culture sites was investigated in four studies,278, 280, 90 _{the number of colonies on a plate in two,}103, 280 _{a combination of number of}

colonies and positive sites in one,279 _{GBS colony-forming units (CFU) in three,}281, 284, 285 _the

number of hours by which a rapid slide coagglutination test identified GBS in two282, 283 _and

selective versus standard culture in one.276 _{The statistical results of the studies where they}

were provided or calculated, are shown in Table 11. Number of colonies per plate

There were three studies reporting on the numbers of colony counts on a plate and, generally, they found that the risk of GBS transmission and EOGBS rises with the number of colonies per plate. Hoogkamp-Korstanje et al. (1982) found that heavy colonisation (87% transmission) was associated with GBS transmission more often than moderate (50% transmission) or light (30% transmission) colonisation.280 _{Heavy colonisation was defined as}

greater than 50 colonies, moderate as 10 to 50 colonies, and light as less than 10 colonies using selective culture on nose, throat, vagina, cervix, rectum and mid-stream urine swabs in labour. Easmon et al. (1985) defined presence of GBS colonies only on enriched culture medium, fewer than 10 colonies on direct plating, 10 to 50 colonies on direct plating, and greater than 50 colonies on direct plating.103 _{They reported the bacterial load results separately for rectal}

and vaginal swabs. However, the labelling of the data in the paper was unclear and could not be interpreted.

Gerards et al. (1985) combined the number of colony counts with the number of sites and created the following criteria: heavy colonisation as three or more sites with more than 50 colonies per plate, moderate colonisation as fewer than three sites with more than 50 colonies or three or more sites with less than 10 or 10 to 50 colonies and light colonisation as fewer than three sites with less than 10 or 10 to 50 colonies.279 _{Among the eight infants with heavy}

colonisation, 50% (n=4) had EOGBS, 50% (n=4) had probable sepsis (but no confirmatory culture from a normally sterile site), and none had asymptomatic colonisation. Among the 35 neonates with moderate colonisation 42.8% (n=15) had EOGBS, 31.4% (n=11) had probable sepsis and 25.7% (n=9) had asymptomatic colonisation. Among the 44 neonates with light colonisation 4.5% (n=2) had EOGBS, 9.1% (n=4) had probable sepsis, and 86.4% (n=38) were asymptomatically colonised. They also found that neonates colonised with more than 50 colonies of GBS were more likely to have EOGBS than neonates with less than 50 colonies. Sites swabbed were nose, throat, external auditory meatus (canal), eyes, umbilicus, skin and rectum immediately after admission to NICU.

Number of colonised sites

Equally, there were three studies reporting the number of colonised sites, and they found that the risk of GBS vertical transmission and EOGBS increased with a larger number of colonised sites. Hoogkamp-Korstanje et al. (1982) compared the risk of GBS vertical transmission in women with one (light) versus two or more (heavy) colonised sites.280 _{The sites swabbed were}

vagina, cervix, rectum, midstream urine, throat and nose. Women with heavy colonisation were 2.5 times more likely to have a neonate with GBS than women with light colonisation (91% vs 36%, RR calculated from percentages given 2.53 95% CI 1.93 to 3.31). Two studies compared the association of one to two colonised sites (light) versus three to four (heavy) colonised sites in neonates, and all found a higher risk of EOGBS in neonates with heavy compared with light colonisation (Pass et al., 1979: 8% [n=7/91] vs 0.5% [n=1/199], RR 15.31 95% CI 1.91 to 122.60; Dillon et al., 1987: 5% [n=20/403] versus 0.4% [n=4/1045], RR 12.97 95% CI 4.46 to 37.70).278, 90 _{Sites swabbed in these studies were external canal, umbilicus,}

throat and anus within one to two hours of birth90 _{and external canal, umbilicus, oropharynx}

Colony forming units (CFU)

There were three studies that investigated the CFU of GBS agreeing that the risk of vertical GBS transmission and EOGBS increases with CFU of GBS.281, 284, 285 _{Jones et al. (1994)}

plotted the CFU of GBS in mothers’ vaginas against CFU of GBS in neonates’ rectum and found a linear correlation (p<0ꞏ001).281 _{In addition, the authors found that mothers’ swabs had}

to contain a minimum of 102 _{GBS for the neonate’s swab to yield a positive result. Finally,}

they found that neonates colonised with ≥105 _{GBS on a rectal swab were delivered by mothers}

colonised with ≥3 x 104 _{GBS on a vaginal swab. On the other hand, the CFU of GBS of}

mothers’ vaginal swabs correlated poorly with neonates’ umbilical and nasopharyngeal cultures. In this study, three neonates developed EOGBS: two had blood culture positive sepsis and one was rectal culture positive and had respiratory distress. All three of the neonates had mothers that were heavily colonized (7.70x106_{, 6.62x10}7_{, and 2.5x106), however, only}

two of the neonates were heavily colonized themselves (7.02x105_{, 5.25x10}6_{). One neonate}

with blood culture positive sepsis was lightly colonised (<101_{). The authors explained that this}

neonate may have been cleaned before culture.

Similarly, Sensini et al. (1997) defined light colonisation as 102 _{to 10}6 _{CFU/GBS ml and heavy}

colonisation as ≥106 _{CFU/GBS ml, finding that mothers with heavy colonisation were more}

likely to transmit GBS to their neonates (50% [n=74/148] vs 30% [n=34/112] RR 1.65 95% CI 1.19 to 2.28).285 _{Only one neonate developed EOGBS and the mother had light}

colonisation. Likewise, Persson et al. (1986) investigated CFU/GBS ml in the urine of mothers, and found that those with ≥104 _{CFU/GBS ml were six times more likely to transmit}

GBS to their neonates compared with mothers with <104 _{CFU/GBS ml (67% [n=6/9] vs 11%}

[n=6/55] RR 6.11 95% CI 2.52 to 14.81).284

Other definitions

Morales et al. (1986, 1987) examined bacterial load by a rapid slide coagglutination test and categorised colonisation as heavy if agglutination with GBS antigens was detectable within five hours of swab or light if agglutination was negative at five hours but positive at 20 hours.282, 283 _{They found that heavily colonised mothers in labour who gave birth to term}

infants were twice times as likely to transmit GBS to their neonates as mothers who were lightly colonised (80% [n=24/30] vs 36% [n=35/98] RR 2.24 95% CI 1.63 to 3.09). Heavily colonised mothers who gave birth to preterm infants were thrice as likely to transmit GBS to their neonates as lightly colonised mothers (73% [n=8/11] vs 24% [n=9/37] RR 2.99 95% CI 1.52 to 5.87).282 _{In 1986, Morales et al. found three cases of neonatal GBS sepsis (positive}

body fluid culture) in term births, all of whom had heavily colonised mothers. In 1987, the group found that GBS sepsis in preterm neonates (including culture of blood, cerebrospinal fluid [CSF], urine and oropharynx cultures with radiographic and clinical signs of infection) was four times more likely in mothers colonised with heavy bacterial load compared with mothers colonised with light load (64% [n=7/11] vs 16% [n=6/37] RR 3.92 95% CI 1.66 to 9.25).

Finally, Boyer et al. (1983) categorised degree of colonisation as heavy if intrapartum vaginal culture was positive on direct plate as well as selective culture, moderate if intrapartum vaginal culture was positive on selective culture only, and light if intrapartum vaginal culture was negative but postpartum rectal or vaginal culture was positive.276 _{Neonatal colonisation was}

3.29 times more likely in heavily compared with light or moderately colonised mothers (64% [n=69/107] vs 20% [20/102] RR 3.29 2.17 to 4.99). Of the women who transmitted GBS to their infants, heavily colonised women were more likely to have neonates colonised at multiple sites (55%) compared with moderately or lightly colonised women (30%, p=0.04). Neonatal sites swabbed were throat, umbilicus, rectum, external ear and nasogastric aspirate. The authors reported four neonates with EOGBS, and all of the mothers were heavily colonised.