Commensal and Pathogenic Microbial Flora

CommensalandPathogenicMicrobial Florain Humans

Medical microbiology is the study of interactionsbetween humans and microorganisms such asbacteria, viruses, fungi and parasites.Although the primary interest is in diseases caused bythese interactions, it must be also appreciated thatmicroorganisms play a critical role in human survival.The normal microflora participates in the metabolismof food products, provides essential growth factors,protects against infections with highly virulent bacteria,and stimulates the immune system.In the absence of bacterial microflora, life as we knowit would be impossible.

The microbial flora is determined by a variety offactors:– age– diet– hormonal state– health– personal hygieneThe human fetus lives in a protected, sterileenvironment, the newborn is exposed to microbesfrom the mother and environment.The infant´s skin is colonized first, followed by theoropharynx, gastrointestinal tract, and other mucosalsurfaces.

Throughout the life of an individual, the microbialpopulation continues to change.For example, hospitalization can lead to thereplacement of normally avirulent bacteria in theoropharynx with gram-negative rods, e.g.Pseudomonas aeruginosa or Klebsiella pneumoniae,that can invade the lungs and cause pneumonia.The growth of Clostridium difficile in thegastrointestinal tract is controlled by the other bacteriapresent in the intestines. In the presence of antibiotics,normal (susceptible) bacteria are eliminated and C.difficile is able to proliferate and producegastrointestinal disease.

Exposure of an individual to bacteriacan lead to one of three outcomes:The bacteria can transiently colonize theperson.The bacteria can permanently colonize theperson.The bacteria can produce disease.

It is important to understand thedistinction between colonization anddisease.Some medical workers use the terminfection inappropriately as asynonym for both terms.

An understanding of medical microbiologyrequires knowledge not only of the differentclasses of bacteria but also of their propensityfor causing disease.Strict pathogens:– Mycobacterium tuberculosis,Neisseria gonorrhoeae, Francisella tularensis,Plasmodium spp., rabies virusOpportunistic pathogens:– e.g. bacteria that are typically members of thehuman ´s normal microflora (Staphylococcusaureus, Escherichia coli and other)

Mouth, oropharynx, nasopharynxThe upper respiratory tract is colonized withnumerous bacteria, with 10 to 100 anaerobesfor every aerobic bacterium.The most common anaerobic bacteria arePeptostreptococcus, Veillonella, Actinomycesand Fusobacterium species.The most common aerobic bacteria areStreptococcus, Haemophilus and Neisseriaspecies.

EarThe most common microorganismcolonizing the outer ear iscoagulase-negativeStaphylococcus species.

EyeThe surface of the eye is colonizedwith coagulase-negativestaphylococci as well as rare numbersof bacteria found in the nasophyranyx(e.g. Haemophilus sp.,Neisseria sp.and viridans streptococci).

Lower respiratory tractThe larynx, trachea, bronchioles andlower airways are generally sterile,although transient colonization withsecretions of the upper respiratorytract may occur after aspiration.

Gastrointestinal tractThe gastrointestinal tract is colonized withmicrobes at birth and remains the home for adiverse population of microorganimsthroughout the life of the host.Although the opportunity for colonization withnew bacteria occurs daily with the ingestionof food and water, the population remainsrelatively constant.Some factors can lead to change of normalmicroflora, e.g. using of antibiotics.

Genitourinary tractIn general, the anterior urethra and vagina arethe only anatomic areas of the genitourinary tractsystem permanently colonized with microbes.Although the urinary bladder can be transientlycolonized with bacteria migrating upstream fromthe urethra, these should be cleared rapidly bythe bactericidal activity of the uroepithelial cellsand flushing action of voided urine.The other structures of the urinary system shouldbe sterile except when disease or an anatomicabnormality is present.

SkinAlthough many microorganisms come intocontact with skin surface, this relatively hostileenvironment does not support the survival ofmost bacteria.Gram-positive bacteria (e.g. coagulase-negativestaphylococci and, less commonly,Staphylococcus aureus, corynebacteria andpropionibacteria) are the most commonmicroorganisms found on the skin surface.Gram-negative bacteria do not permanentlycolonize the skin surface, because the skin is toodry.