Normal Flora: Resident and Transient Types

Introduction to Normal Flora

• Normal flora, also known as microbiota, encompasses bacteria and fungi that reside permanently in specific body areas like the skin, oropharynx, colon, and vagina.
• Viruses and parasites are not considered part of the normal flora, even if present in asymptomatic individuals.
• The central nervous system (CNS), blood, lower bronchi and alveoli, liver, spleen, kidneys, and bladder are typically free of organisms, except for occasional transient ones.

Resident Flora

• Resident flora comprises species that live in specific sites on or in nearly all individuals nearly all the time.
• These flora are typically harmless or even beneficial in their natural sites.
• Resident flora can become pathogenic if introduced into abnormal sites; for example, E. coli in the urinary bladder can cause cystitis, acting as an opportunist.

Transient Flora

• Transient flora are species found periodically in the body but are not as well-adapted as resident flora.
• Streptococcus pneumoniae, for example, is a transient in the upper respiratory tract that typically doesn't cause harm in healthy individuals.
• Transients can become pathogenic when the host's resistance is lowered; S. pneumoniae may cause serious pneumonia in elderly people with influenza.
• Transient flora may be present for hours, days, or weeks, derived from the environment, and do not permanently establish themselves on the body's surface.

Role of Resident Flora: Advantages

• Resident flora supports host nutrition and prevents colonization by pathogens.
• They compete for receptors or binding sites and nutrients.
• Resident flora produces antibiotics or bacteriocins and toxic products harmful to pathogens.
• They stimulate natural antibodies that cross-react with pathogenic organisms.
• Resident flora colonisation prevent or suppress pathogen invasion.
• Intestinal flora synthesizes vitamins, especially Vitamin K, and aids nutrient absorption.
• Antibodies cross-react with pathogens sharing related antigens, raising the host's immune status.
• Bacteriocins produced by microbes have a harmful effect on pathogens.
• Endotoxins can trigger the Alternate Complement Pathway.

Role of Resident Flora: Disadvantages

• Normal flora may cause disease under certain circumstances, such as bloodstream invasion.
• Streptococcus viridans can be introduced into the bloodstream following tooth extraction or tonsillectomy.
• Bacteroides from the large intestines can cause peritoneal cavity infections.
• Members of the resident flora found in disease are considered opportunists.
• Opportunistic infections can especially impact immunocompromised or debilitated individuals.
• Bacteroides fragilis is an important cause of peritonitis associated with intestinal wall perforation.
• S. mutans, if introduced into the bloodstream, may cause infective endocarditis.
• S. mutans may also cause dental caries.
• Penicillinase-producing organisms can interfere with therapy.
• Broad-spectrum antibiotics are likely to inhibit the normal flora.
• Normal flora can cause confusion in diagnosis due to their ubiquitous presence and resemblance to some pathogens.

Function of Normal Flora

• Normal flora inhibits the growth of pathogens by producing antibacterial chemicals and competing.

Probiotics

• Probiotics are live, nonpathogenic bacteria that may be effective in treating or preventing human diseases.
• Probiotics provide colonization resistance, enhance the immune response, and reduce the inflammatory response against pathogens.

Uses of Probiotics

• Probiotics treat and prevent diarrhea, including rotavirus diarrhea in children, traveler's diarrhea, and post-antibiotic diarrhea.
• Probiotics alleviate symptoms of lactose intolerance and food allergies like milk allergies.
• Probiotics help inflammations like Crohn's disease.
• Probiotics can treat inflammatory diseases, may prevent treat cancer, can help with immunomodulation(arthritis), treat respiratory diseases(cystic fibrosis ingesting Lactobacillus)

Harmful Effects of Normal Flora

• If imbalanced, they disrupt mucosa, cause disease in immunocompromised individuals, or cause disease in new locations (gingivitis and dental caries).

Factors Altering Normal Flora

• Diet, antibiotics use, disease state, and moisture levels can impact the normal flora.

Effect of Antibiotics

• Antibiotics can diminish resident flora.
• Antibiotics do not differentiate between pathogens and resident bacteria, leading to yeast or pathogenic bacteria overgrowth and new infections.
• Clindamycin can suppress normal flora, allowing Clostridium difficile to overgrow and cause severe colitis.
• Neomycin for GI surgery sterilizes the gut temporarily, with levels returning to normal after several days.

Localizations of the Normal Flora

• Skin, eyes (conjunctiva), nose and throat (upper respiratory tract), mouth, large intestine, and urinary and reproductive systems are homes to the normal flora.

Normal Flora of the Skin

• Staphylococcus epidermidis is typically non-pathogenic but can cause disease in certain sites such as artificial heart valves.
• Staphylococcus aureus is located superficially in the stratum corneum and hair follicles, acting as a reservoir to replenish the superficial flora after handwashing.
• Propionibacterium is found in deep follicles where oxygen tension is low.
• Propionibacterium acnes is a common skin anaerobe that may contribute to acne, which is a gram-positive bacillus, anaerobic, and lipophilic.
• Candida albicans can enter the bloodstream through skin punctures and cause systemic infections in individuals with reduced cell-mediated immunity.

Normal Flora of the Respiratory Tract

• The nose is heavily colonized with coagulase-negative Staphylococcus (CONS), corynebacteria, and S. aureus in 20-30% of people.
• The nasopharynx hosts bacterial species, including potentially pathogenic strains like N. meningitidis, S. pneumoniae, S. pyogenes, and H. influenzae.
• The throat contains a mixture of Staphylococcus epidermidis, Neisseria species, and streptococcus Viridans.
• Trachea, bronchi, and lung tissues are sterile due to mucus, phagocytosis by alveolar macrophages, and lysozyme in nasal mucus.

Normal Flora of the Intestinal Tract

• The stomach of fasting people has few microbes due to low pH and enzymes.
• Small intestine's terminal ileum contains streptococci, lactobacilli, and yeasts (Candida albicans). Colon's contents include Bacteroides (B. fragilis), Bifidobacterium, Eubacterium, Coliforms, Enterococcus (E. faecalis), Lactobacillus, and Clostridium (C. perfringens).

Diseases caused by Normal flora of intestinal tract

• E. Coli can cause UTI.
• Bacteroides fragilis can cause Peritonitis (perforation of intestinal).
• Enterococcus feacalis can cause UTI & endocarditis.
• Pseudomonas Aeruginosa can cause various infections in patients with low host defenses.

Normal Flora of the Urinary and Reproductive Systems

• The urethra in both sexes is colonized by S. epidermidis, Strep. faecalis, and diphtheroids.
• The vagina's flora composition changes with age.
• Before puberty the composition contains Staphylococcus, Streptococci, Diphtheroids and E. coli.
• Subsequently, Lactobacillus aerophilus maintains an acid pH.
• Trichomonas vaginalis and Candida are present in healthy individuals.
• Lactobacilli species predominate in normal females, and produce acid to maintain pH.
• High vaginal pH and low estrogen results in low Lactobacilli presence.

Diseases caused by Normal flora of Genitourinary tract

• Antibiotics lead to yeast-like overgrowth, resulting in "Candida vaginitis".
• Recurrent UTIs are cause by E. Coli.
• 15-20% of child bearing aged women carrying group B streptococci is likely to result in new born sepsis. S. aureus in Vagina leads to toxic shock syndrome.
• Infections and nonhemolytic bacteria contaminates urethra. Mycobacterium smegmatis in uncircumcised men secretes around area. Staph leads to UTI too.