Investigations At The Subspecies Level

One aspect of gut microbiology that is not amenable to current probing or PCR-based methodologies is subspecies differentiation (i.e., investigations of the microbial complexity and dynamics below the phylogenetic level of species). A number of studies have, however, demonstrated the importance of such research (48,51,52). One study monitored the composition of the bifidobacterial and lactobacilli populations of two healthy humans over a 12-month period (52). Overall, the bifidobacterial levels of both individuals were relatively stable throughout the study period [ ~ 1010/g of feces (wet weight)]. Lactobacilli levels were relatively constant in subject one (~109/g) but fluctuated considerably in samples from subject two (106-109 per gram). Bacteroides and enterobacterial levels were also examined during the study; the former remained stable for both individuals and the latter displayed marked variability (especially in subject two, for whom numbers were below the detection limit in weeks 17 and 20). Genetic fingerprinting techniques were used to differentiate the predominant bacterial isolates of the bifidobacterial and lactobacillus populations. Interestingly, two distinct bifidobacterial profiles were seen, with one individual harboring a simple, stable bifidobacterial population (five distinct strains of bifidobacteria were detected during 12 months, one of which was numerically predominant throughout the study). In contrast, the second subject harbored a more complex and dynamic bifidobacteria! population (36 distinct strains were seen over the

12 months, with between four and nine strains at any given time). The Lactobacillus populations of both subjects were simple and stable. None of the lactic acid bacterial strains isolated during the study were common to both individuals, further accentuating the inter-individual variations of these two microbial ecosystems (52). Subsequent work from the same laboratory extended these investigations to a further eight healthy humans (four males, four females) (51). Two separate samples were collected and processed for each individual. The results of this secondary study confirmed the findings of the former, with bifidobacteria! levels remaining relatively stable and Lactobacillus numbers varying greatly. Again, each individual harbored unique bifidobacterial and Lactobacillus strains (at least in regard to the numerically predominant microbiota). Intriguingly, half of the subjects were shown to harbor a complex bifidobacteria microbiota (five or more predominant strains).

McBurney and colleagues (48) examined the perturbation of the enterobacterial populations of the initial long-term study by McCartney and colleauges (52). Similar to the bifidobacterial populations of these two individuals, the enterobacterial population of subject one was relatively simple and stable (predominated by a single strain) whilst subject two harbored a diverse and dynamic enterobacterial biota (27 distinct strains were identified over 12 months). As mentioned previously, enterobacterial levels were below detection for subject two during weeks 17 and 20. This individual undertook a 7-day course of amoxicillin for a respiratory infection during weeks 21 and 22, after which time the enterobacterial population re-emerged. Most interesting, though, was the antibiotic-resistance profiles of this bacterial group before and after treatment. Strains isolated prior to antibiotic administration were susceptible to a wide range of antibiotics tested, whereas strains isolated following treatment were resistant to a number of antibiotics. Thirteen weeks after amoxicillin administration, multiple drug-resistant enterobacterial strains were still present. In the following 2 months, strains resistant to ampicillin were still harbored, and only after 25 weeks post-treatment did the predominant enterobacterial microbiota return to a simple, stable, susceptible composition.

Taken together, the above-mentioned work clearly demonstrates the value of investigations at the subspecies level, as such studies afford more detailed analysis of the diversity and dynamics of the gut microbiota. Furthermore, such strategies allow the detection of microbial perturbations which are often not evident at the bacterial group, genus or species levels (79).

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Pregnancy And Childbirth

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