Normal bacterial flora
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The human body hosts a diverse population of microorganisms, collectively known as the normal flora or microbiota. These microorganisms, predominantly bacteria, colonise various body sites and play crucial roles in maintaining health.
Protective roles of normal flora:
- Competition for colonisation sites: Normal flora occupy niches on body surfaces, effectively competing with potential pathogens for attachment sites and nutrients. This competitive exclusion helps prevent colonisation by harmful microorganisms.
- Production of bacteriocins: Certain members of the normal flora produce bacteriocins—proteinaceous toxins that inhibit the growth of other bacterial strains. Bacteriocins can inhibit many disease-causing bacteria, including some antibiotic-resistant strains, suggesting their potential role in antagonising pathogenic infections.
Bacteriocins
Bacteriocins are antimicrobial peptides produced by bacteria to inhibit the growth of closely related or competing bacterial species. They function primarily by disrupting bacterial cell membranes, interfering with essential cellular processes, or degrading cell wall components. Unlike broad-spectrum antibiotics, bacteriocins are typically narrow-spectrum, targeting specific bacterial strains. Their production is often regulated by quorum sensing and environmental conditions.Impact of antibiotics on normal flora:
Antibiotic use can disrupt the balance of the normal flora with the following effects:- Suppression of beneficial microorganisms: Antibiotics can reduce microbial diversity and alter the functional attributes of the microbiota, leading to a negative impact on host health. Examples include antibiotic induced death of intestinal bacteria that synthesise vitamin K leading to vitamin K deficiency.
- Overgrowth of opportunistic pathogens: Disruption of the normal flora can favour infection by exogenous pathogens and overgrowth by endogenous pathogens. Examples include overgrowth of Clostridium difficile, leading to pseudomembranous colitis.
Summary of normal flora by body site:
| Body Site | Normal Flora |
|---|---|
| Nasopharynx | Streptococci, Staphylococci, Haemophilus, Neisseria, Corynebacteria, Moraxella, Mixed anaerobes, Candida, Actinomyces |
| Oral Cavity | Streptococcus mutans, Streptococcus sanguinis, Actinomyces, Lactobacillus, Veillonella, Fusobacterium, Candida species |
| Skin | Staphylococci, Streptococci, Corynebacteria, Propionibacteria, Yeasts |
| Bowel | Enterococcus spp., Escherichia coli, Bacteroides spp., Bifidobacterium spp., Clostridium spp., Lactobacillus spp., Peptostreptococcus spp., Fusobacterium spp., Prevotella spp., Klebsiella spp., Proteus spp., and Candida |
| Vagina | Lactobacilli, Staphylococci, Streptococci, Corynebacteria, Candida, Actinomyces, Mycoplasma hominis |