NORMAL FLORA

DR A. R. OJEWUYI

LECTURER/ CONSULTANT MEDICAL MICROBIOLOGIST

What is Normal Microbial Flora

• Normal microbial flora, or normal flora are a variety of microorganisms that inhabit the skin and mucous membranes of healthy normal persons.

• These organisms can either be resident or transient.

• The resident flora consists of relatively fixed types of microorganisms regularly found in a given area at a given age; if disturbed, it promptly reestablishes itself.

• The transient flora consists of nonpathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours, days, or weeks; it is derived from the environment, does not produce disease, and does not establish itself permanently on the surface.

• Members of the transient flora are generally of little significance so long as the normal resident flora remains intact.

• However, if the resident flora is disturbed, transient microorganisms may colonize, proliferate, and produce disease.

Origin of Microbial Flora• In a normal pregnancy, the foetus is in a sterile

environment until birth.

• From the first day of life, the newborn acquires different microorganisms from the environment.

• The organisms then localise to the to different sites they find most suitable.

• They colonize various anatomic sites and become the predominant organisms.

• Once established, the microorganisms can develop a relationship with the host: symbiosis, commensalism or parasitism.

• Symbiosis is a biological relationship between two or more organisms where both the host and organism benefit from the relationship.

• Also called mutualism.

• Example is Lactobacilli in the urogenital tract of women, gives the host protection from pathogenic species while deriving nutrients from the host.

• In commensalism, the organism benefits with no harmful or beneficial effect to the host.

• An example is Proteus mirabilis in the gastrointestinal tract of man.

• In Parasitism, the microbe benefits at the expense of the host.

• An example is a pathogenic intestinal parasites Entamoeba histolytical which causes amoebic dysentery and intestinal ulcers.

Characteristics of Normal Flora

• Different body sites may have same or different flora, depending on local conditions.

• Local conditions select for those organisms that are suited for growth in a particular area.

• Some pathogenic organisms may establish themselves in a host without manifesting symptoms, the host is then referred to as a carrier.

• A carrier state could either be acute(transient) or chronic (permanent).

• The normal microbial flora primes the development of immunologic response in the host thus play a significant role in the development of host resistance to infections.

• The microbial flora also produce conditions at the micro-environmental level that block colonization by extraneous pathogens, thus when the composition of the indigenous flora is altered (e.g. by use antibiotics), other organisms capable of causing disease may fill the void.

• The resident flora of certain areas plays a definite role in maintaining health and normal function.

• Members of the resident flora in the intestinal tract synthesize vitamin K and aid in the absorption of nutrients.

• On mucous membranes and skin, the resident flora may prevent colonization by pathogens and possible disease through ’bacterial interference’.

• The mechanism of bacterial interference may involve competition for receptors or binding sites on host cells, competition for nutrients, mutual inhibition by metabolic or toxic products, mutual inhibition by antibiotic materials or bacteriocins, or other mechanisms.

• Suppression of the normal flora clearly creates a partial local void that tends to be filled by organisms from the environment or from other parts of the body.

• Such organisms behave as opportunists and may become pathogens.

• The normal flora rarely cause disease in immune competent individuals unless when dislodge from its usual site to another part of the body where it can cause disease.

• However, these organisms may cause significant and life threatening infections or exacerbate existing infections in individuals lacking a fully responsive immune system e.g. patients with diabetes, chronic renal failure, malignancies and HIV/AIDS.

• The resulting infections from the flora in these patients are then called opportunistic infections.

• For example, streptococci of the viridans group are the most common resident organisms of the upper respiratory tract.

• If large numbers of them are introduced into the bloodstream (e.g., following tooth extraction or tonsillectomy), they may settle on deformed or prosthetic heart valves and produce infective endocarditis.

• Small numbers occur transiently in the bloodstream with minor trauma (e.g., dental scaling or vigorous brushing).

• Normal flora of the skin can also cause severe infection if introduced into the bloodstream during intravenous cannulation, urinary tract infections after urethral catherization and wound sepsis after surgical procedures.

• Bacteroides species are the most common resident bacteria of the large intestine and are quite harmless in that location. If introduced into the free peritoneal cavity or into pelvic tissues along with other bacteria as a result of trauma, they cause suppuration and bacteremia.

• There are other examples, but the important point is that microbes of the normal resident flora are harmless and may be beneficial in their normal location in the host and in the absence of coincident abnormalities.

• They may produce disease if introduced into foreign locations in large numbers and if predisposing factors are present.

Factors That Determine the Composition Microbial Flora

• The composition of the flora at a body site is influenced by the nutritional and environmental factors such as amount and types of nutrients available at the site.

• The affinity of microorganisms for a specific site also depends on the ability of those organisms to resist the antibacterial effects of substances such as bile, lysozyme, or fatty acids.

• The composition of microbial flora is also affected by pH.

• The environmental conditions change with age, nutritional status, disease states and drug or antibiotic effects.

• These changes can then predispose to opportunistic infections.

EXAMPLES OF BACTERIAL FLORA

SKIN

Staphylococcus epidermidis

Staphylococcus aureus (in small numbers)

Micrococcus species

Nonpathogenic Neisseria species

Alpha-hemolytic and nonhemolytic streptococci

Diphtheroids

Propionibacterium species

Peptostreptococcus species

Small numbers of other organisms (Candida species, Acinetobacter species, etc)

NASOPHARYNX

Any amount of the following: diphtheroids, nonpathogenic Neisseria species, -hemolytic streptococci; S epidermidis, nonhemolytic streptococci, anaerobes (too many species to list; varying amounts of Prevotella species, anaerobic cocci, Fusobacterium species, etc)

Lesser amounts of the following when accompanied by organisms listed above: yeasts, Haemophilus species, pneumococci, S aureus, gram-negative rods, Neisseria meningitidis

EXAMPLES OF BACTERIAL FLORA

GASTROINTESTINAL TRACT AND RECTUM

Various Enterobacteriaceae except Salmonella, Shigella, Yersinia, Vibrio, and Campylobacter species

Non-dextrose-fermenting gram-negative rods

Enterococci

Alpha-hemolytic and nonhemolytic streptococci

Diphtheroids

S. aureus in small numbers

Yeasts in small numbers

Anaerobes in large numbers (too many species to list)

GENITALIAAny amount of the following: Corynebacterium species, Lactobacillus species, -hemolytic

and nonhemolytic streptococci, nonpathogenic Neisseria species

The following when mixed and not predominant: enterococci, Enterobacteriaceae and other gram-negative rods, S epidermidis, Candida albicans, and other yeasts

Anaerobes (too many to list); the following may be important when in pure growth or clearly predominant: Prevotella, Clostridium, and Peptostreptococcus species

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