CommensalCommensaland and

PathogenicPathogenicMicrobial Flora Microbial Flora

in Humansin Humans

Medical microbiology is the study of interactions between humans and Medical microbiology is the study of interactions between humans and microorganisms such as bacteria, viruses, fungi and parasites.microorganisms such as bacteria, viruses, fungi and parasites.

Although the primary interest is in diseases caused by these interactions, it Although the primary interest is in diseases caused by these interactions, it must be also appreciated that microorganisms play a critical role in human must be also appreciated that microorganisms play a critical role in human survival.survival.

The normal microflora participates in the metabolism of food products, The normal microflora participates in the metabolism of food products, provides essential growth factors, protects against infections with highly provides essential growth factors, protects against infections with highly virulent bacteria, and stimulates the immune system.virulent bacteria, and stimulates the immune system.

In the absence of bacterial microflora, life as we know it would be In the absence of bacterial microflora, life as we know it would be impossible.impossible.

The microbial flora is determined by a variety of factors:The microbial flora is determined by a variety of factors:– ageage– dietdiet– hormonal statehormonal state– healthhealth– personal hygienepersonal hygiene

The human fetus lives in a protected, sterile environment, the The human fetus lives in a protected, sterile environment, the newborn is exposed to microbes from the mother and environment.newborn is exposed to microbes from the mother and environment.

The infant´s skin is colonized first, followed by the oropharynx, The infant´s skin is colonized first, followed by the oropharynx, gastrointestinal tract, and other mucosal surfaces.gastrointestinal tract, and other mucosal surfaces.

Throughout the life of an individual, the microbial Throughout the life of an individual, the microbial population continues to change.population continues to change.

For example, hospitalization can lead to the For example, hospitalization can lead to the replacement of normally avirulent bacteria in the replacement of normally avirulent bacteria in the oropharynx with gram-negative rods, e.g. oropharynx with gram-negative rods, e.g. Pseudomonas aeruginosaPseudomonas aeruginosa or or Klebsiella Klebsiella pneumoniaepneumoniae, that can invade the lungs and cause , that can invade the lungs and cause pneumonia.pneumonia.

The growth of The growth of Clostridium difficileClostridium difficile in the in the gastrointestinal tract is controlled by the other gastrointestinal tract is controlled by the other bacteria present in the intestines. In the presence of bacteria present in the intestines. In the presence of antibiotics, normal (susceptible) bacteria are antibiotics, normal (susceptible) bacteria are eliminated and eliminated and C. difficileC. difficile is able to proliferate and is able to proliferate and produce gastrointestinal disease. produce gastrointestinal disease.

Exposure of an individual to bacteria Exposure of an individual to bacteria can lead to one of three outcomes:can lead to one of three outcomes:

The bacteria can transiently colonize the The bacteria can transiently colonize the person.person.

The bacteria can permanently colonize the The bacteria can permanently colonize the person.person.

The bacteria can produce disease.The bacteria can produce disease.

It is important to understand the It is important to understand the distinction between colonization and distinction between colonization and disease.disease.

Some medical workers use the term Some medical workers use the term infection inappropriately as a infection inappropriately as a synonym for both terms.synonym for both terms.

An understanding of medical microbiology An understanding of medical microbiology requires knowledge not only of the different requires knowledge not only of the different classes of bacteria but also of their classes of bacteria but also of their propensity for causing disease.propensity for causing disease.

Strict pathogens:Strict pathogens:– Mycobacterium tuberculosis,Mycobacterium tuberculosis,

Neisseria gonorrhoeae, Francisella tularensis, Neisseria gonorrhoeae, Francisella tularensis, PlasmodiumPlasmodium spp., rabies virus spp., rabies virus

Opportunistic pathogens:Opportunistic pathogens:– e.g. bacteria that are typically members of the e.g. bacteria that are typically members of the

human ´s normal microflora (human ´s normal microflora (Staphylococcus Staphylococcus aureus, Escherichia coliaureus, Escherichia coli and other) and other)

Mouth, oropharynx, nasopharynxMouth, oropharynx, nasopharynxThe upper respiratory tract is colonized with The upper respiratory tract is colonized with numerous bacteria, with 10 to 100 anaerobes for numerous bacteria, with 10 to 100 anaerobes for every aerobic bacterium.every aerobic bacterium.

The most common anaerobic bacteria are The most common anaerobic bacteria are Peptostreptococcus, Veillonella, ActinomycesPeptostreptococcus, Veillonella, Actinomyces and and FusobacteriumFusobacterium species. species.

The most common aerobic bacteria are The most common aerobic bacteria are Streptococcus, HaemophilusStreptococcus, Haemophilus and and Neisseria Neisseria species.species.

EarEarThe most common microorganism The most common microorganism colonizing the outer ear is colonizing the outer ear is coagulase-negative coagulase-negative StaphylococcusStaphylococcus species. species.

EyeEyeThe surface of the eye is colonized The surface of the eye is colonized with coagulase-negative staphylococci with coagulase-negative staphylococci as well as rare numbers of bacteria as well as rare numbers of bacteria found in the nasophyranyx (e.g. found in the nasophyranyx (e.g. HaemophilusHaemophilus sp., sp., NeisseriaNeisseria sp. and sp. and viridans streptococci).viridans streptococci).

Lower respiratory tractLower respiratory tract

The larynx, trachea, bronchioles and The larynx, trachea, bronchioles and lower airways are generally sterile, lower airways are generally sterile, although transient colonization with although transient colonization with secretions of the upper respiratory tract secretions of the upper respiratory tract may occur after aspiration.may occur after aspiration.

Gastrointestinal tractGastrointestinal tractThe gastrointestinal tract is colonized with The gastrointestinal tract is colonized with microbes at birth and remains the home for a microbes at birth and remains the home for a diverse population of microorganims diverse population of microorganims throughout the life of the host.throughout the life of the host.

Although the opportunity for colonization with Although the opportunity for colonization with new bacteria occurs daily with the ingestion of new bacteria occurs daily with the ingestion of food and water, the population remains food and water, the population remains relatively constant.relatively constant.

Some factors can lead to change of normal Some factors can lead to change of normal microflora, e.g. using of antibiotics.microflora, e.g. using of antibiotics.

Genitourinary tractGenitourinary tractIn general, the anterior urethra and vagina are the In general, the anterior urethra and vagina are the only anatomic areas of the genitourinary tract only anatomic areas of the genitourinary tract system permanently colonized with microbes.system permanently colonized with microbes.

Although the urinary bladder can be transiently Although the urinary bladder can be transiently colonized with bacteria migrating upstream from colonized with bacteria migrating upstream from the urethra, these should be cleared rapidly by the the urethra, these should be cleared rapidly by the bactericidal activity of the uroepithelial cells and bactericidal activity of the uroepithelial cells and flushing action of voided urine.flushing action of voided urine.

The other structures of the urinary system should The other structures of the urinary system should be sterile except when disease or an anatomic be sterile except when disease or an anatomic abnormality is present.abnormality is present.

SkinSkinAlthough many microorganisms come into contact with skin surface, this Although many microorganisms come into contact with skin surface, this relatively hostile environment does not support the survival of most relatively hostile environment does not support the survival of most bacteria.bacteria.

Gram-positive bacteria (e.g. coagulase-negative staphylococci and, less Gram-positive bacteria (e.g. coagulase-negative staphylococci and, less commonly, commonly, Staphylococcus aureusStaphylococcus aureus, corynebacteria and propionibacteria) , corynebacteria and propionibacteria) are the most common microorganisms found on the skin surface. are the most common microorganisms found on the skin surface.

Gram-negative bacteria do not permanently colonize the skin surface, Gram-negative bacteria do not permanently colonize the skin surface, because the skin is too dry.because the skin is too dry.