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Classification of Bacteria &Normal Flora
Dr. Zaheer Ahmed ChaudharyAssociate Professor MicrobiologyDepartment of Pathology
Classification of Bacteria
Bacteria can be classified on the basis of :Nature of cell wallStaining characteristicsAbility to grow in presence or absence of oxygenSpore formation
Classification of Bacteria
Current classification is primarily based on morphology and biochemical characteristics of bacteria.Initial criteria used is the structure of cell wall i.e. it is rigid, flexible or not.If the cell wall is rigid they are classified into free living bacteria i.e. they can grow on lab medium.
Non free living bacteria – which are obligate intracellular parasites and only live in human or animal cells.Free living are further subdivided for shape and staining reactions into gram positivecocci, gram negativecocciand rods, with different oxygen needs&spore forming ability.
Flexible Bacteria
Flexible bacteria – can be with thin wall(e.gSpirocahetes)and without cell wall(Mycoplasma).On the basis of bio-chemical reactions bacteria can be classified into separate genus and species.Current criterion is the base sequence of DNA genome.Several bacteria have beenre-classifiedon this basis.
DNA sequencing has revealed different classification of bacteria, previously classified into other genus.Pseudomonascepaciahas now been re-classified asBurkholderiacepaciabecause the base sequence of DNA was found to be significantly different from the DNA of the members of genus of pseudomonas.
Normal Flora
IntroductionNormal flora is the term used to describe various bacteria or fungi, which are permanent resident of certain body sites like skin, mouth,colon.Viruses and parasites are usually not included in this category.Normal flora are also referred ascommensalsi.e. they drive benefit from the host but do not damage it.
Members of the normal flora are low virulenceorganisms.They are nonpathogenic in their normal anatomical sites and if they leave that site they can cause the disease inimmuno-compromisedpersons.Normal flora vary both in number and kind from each other. Internal body organisms e.g. CNS, blood, bronchi, liver, spleen, kidney, heart and bladder are usually sterile while other body areas show the presence of normal flora.
Important Terminologies
CarrierState:This term implies when a person harbors a potential pathogen and can be a source of infection to others. (Typhoid Marry)Colonization:Is the acquisition of new organisms. It can cause the disease or can get eliminated by host defense. The colonizer can transmit the organisms to others i.e. reservoir of infection.
Normal Flora
Members of normal flora can play role in health maintenance or cause disease in threeways:Basicallythe organisms are non pathogenic in their usualanotomicallocations but can be pathogenic in other parts of thebody likeImmuno-compromised and debilitatedpersons.
Protective host defense mechanism is constituted by normal flora by colonization resistance which limits the growth ofpathogins.Whennormal flora issuppressed,pathogens grow and produce the disease e.g.antibiotics reduce the normal colonic flora and allow clostridiumdifficileto overgrow which is resistant to antibiotics.
Intestinal bacteria produces vitamin B & K. Normal flora serves a nutritional function. The patients who are poorly nourished and treated with oral antibiotics can have deficiency of thesevitamins.
Skin Normal Flora
Staph.epiderimidisis the most predominant bacteria on skin which can involve artificial heart valves and joint prosthesis.Population of organisms is103-104per cm squareIt is located on stratumcorneumlayer of skin which is superficial and in hair follicles.Anaerobic skin organisms arepeptococcus.Proprinibacterium (acne) are located in deeper follicles in the dermis.
Yeast (candidaalbicans) is also present on skin which can enter the blood stream and cause disease in IV catheters and drug users.It is important cause of systemic infections in decreased cell immunity patients.
Mouth, nose, throat is the site of colonization of manyorganisms.Nose is colonized by staphylococcusaureusand streptococcus which result in occasional outbreaks in newborn nursery.Throat contains mixture ofviridansstreptococcus,Neisseriaspecies and staph.epidermidis.
These non pathogenic organisms occupy the attachment sites onpharyangialmucosa and limit growth of pathogenicorganisms.Mouth has big population ofviridansstreptococcus.Strept.mutanscauses dental plaque leading to caries teeth.This produces large amounts of acid whichdemineralizesthe enamel and initiate caries.Viridansstreptis the leading cause ofsubacutebacterialendocarditisafter dental surgery.
Eikenellacorrodenscauses skin and soft tissue infections with human bite and clenched fist injuries.Anaerobic bacteria likebacteroides,prevotella,fusobacterium, clostridium,peptostreptococcusare found in cervices of gums. These organism can cause lungabsessin poor dental hygiene and debilitated patients.Actinomycosisisraeilican causeabscessof jaw, lungs, abdomen.
Stomach contains very few organisms due to acidity. Small intestine has small number of streptococcus, lactobacilli and yeast.Large amount of these organisms are found in terminal ileumColon is the major location for bacteria inGIT.About 20% feces consist of bacteria, 90% beings the anaerobes especiallybacteroidesfragilis. Most abundant facultative bacteria isE.coli.
Normal gut flora plays significant role in extra intestinal diseases e.g.E.colicausing UTI andB.fragiliscausing peritonitis.Other anaerobic bacteria arefusobacterium,peptostreptococcus, enterococcusfaecalis(UTI and endocarditis) andpseudomonoasaeruginosa(hospital acquired infections).
Antibiotics likeclindamycincan suppress the normal flora and allow rare toxin producing organism to grow and cause the disease.Neomycin used to sterilize gut can also reduce the normal flora population.
Genitourinary tract normal flora
Adult women have lactobacillus species responsible for acid production and lowpH.pH increases in menopause and before puberty due to rare presence of lactobacillus which prevents the growth of potential pathogens.Use of antibiotics results in growth ofcandidaspecies leading tocandidavaginitis.
Colonization of vagina can occur due to close approximate of anus byfaecalflora e.g.E.coliandenterococci.15% -20% women carry group B streptococcus in vagina during the child bearing age.Vagina can also be colonized bystaph.aureusin about 5% women leading to toxic shock syndrome
Urine is sterile in healthy persons but can be contaminated bystaph.epidermidis, coliform,diphtheroidsandnonhemolyticstreptococci.Mycobacteriumsmegmatiscan contaminate area around urethra of women and uncircumcisedmen.Staph.saprophyticuscan cause UTI by contamination of skin surroundinggenitourinary tract.





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