Unit 2: Bacteriology
134
Lecture 15 - Normal microbial flora
Do you know that you have 3X as many bacterial cells
as human cells?
If you include fungal and viral cells, only about 25% of
your cells are actually human
The other 75% are MICROBES
The population of microorganisms (bacteria & fungi)
that inhabit the skin & mucous membrane of pharynx,
colon & vagina of healthy normal persons. Viruses &
parasites are not considered as normal flora. Includes 2
groups:
1) The resident flora: consist of fixed types of commensals
of microorganisms reestablishes itself. Play a role in
maintaining health & normal function, in intestine
synthesize vit. K & aid in the absorption of nutrients, in
skin & mucous membrane prevent the colonization of
pathogens through bacterial interference. But normal flora
may produce disease under certain circumstances.
2) The transient flora: consist of nonpathogenic or
potentially pathogens that inhabit skin or mucous
membranes for hours, days or weeks. It is derived from
the environment does not produce disease & does not
establish itself.
N.F of the Skin:
S.epidermidis, S.aureus, Micrococcus, streptococci,
Enterococcus, nonpathogenic N., diphtheroids
(Corynebacterium spp.), Propionibacterium, G -
coliform, fungi & yeast (Candida albicans in skin folds),
nonpathogenic mycobacteria (in external ear & genitalia,
which are rich in sebaceous secretions).
Factors that eliminating nonresident microorganisms from
the skin:
1) Low pH.
2) Lysozyme.
3) Fatty acids in sebaceous secretions.
Anaerobic & aerobic bacteria join to form syngeristic
infections (gangrene, necrotizing fasciitis & cellulitis).
N.F of the Mouth & Upper R.T:
Nose: S.aureus, S.epidermidis, streptococci &
Corynebacteria.
Mucous membrane of mouth & pharynx: at birth
sterile, after 4-12 hrs contaminated from birth canal with
viridans streptococci (as resident flora remain so far life).
Mouth: (early in life) staphylococci, N., Moraxella
catarrhalis, Diphtheroids & lactobacilli.
(When teeth erupt) spirochetes, Fusobacterium spp.,
Prevotella spp., Rothia spp.(G+, pleomorphic aerobes),
Capnocytophaga spp. & lactobacilli.(in adults)
Actinomyces spp.(in tonsillar tissue & gingiva),
yeast(C.albicans) & various protozoa.
Pharynx & trachae: similar flora of mouth.
Bronchi: few bacteria. Small bronchi & alveoli: sterile.
The predominant flora in upper R.T: streptococci,
staphylococci, N., diphtheroids, Hemophilic,
pneumococci, Mycoplasma & Prevotella.
Infections of mouth & upper R.T (by aerobes &
anaerobes): periodontal infection, perioral abscesses &
sinusitis, necrotizing pneumonia, lung abscess &
empyema (by aspiration of saliva containing up to 10
2
of
aerobes & anaerobes).
N.F of the Intestinal Tract:
At birth is sterile.
Newborns (in intensive care): the intestine colonized by
Enterobacteriaceae (K., Citrobacter, Enterobacter).
Breast-feed children: Large No. of lactobacilli, lactic
acid streptococci & Bifidobacterium spp. (G+, nonmotile,
anaerobic bacteria), produce acid from carbohydrates &
tolerate pH 5.
Bottle-feed children: More mixed flora (lactobacilli less
prominent).
Provetella
Fusobacterium
Unit 2: Bacteriology
135
Normal adult:
Esophagus: contains microorganisms arriving with saliva
& food.
Stomach: acidity keeps the No. of microorganisms at
minimum (10
3
-10
5
bacteria/g of contents).Cimetidine (for
peptic ulcer) → increase in microbial flora of the
stomach.
As the pH of intestinal contents becomes alkaline →
resident flora gradually increases. Duodenum (10
3
-10
6
bac./g of contents), Jejunum & Ileum (10
5
-10
8
), Cecum &
transverse colon (10
8
-10
10
), Sigmoid colon & Rectum
(10
11
=10-30% of fecal mass).
Upper Intestine: lactobacilli & enterococci.
Colon: anaerobes (96-99% of resident flora):
Fusobacterium spp., Bifidobacterium, clostridia (10
3
-10
5
bac./g of contents) & anaerobic G+ cocci
(Peptostreptococcus) & facultative aerobes (1-4%)
includes: G- coliform & enterococci & small No. of
Pseudomonas, lactobacilli, Proteus & Candida.
The role of intestinal flora:
1) Synthesis of vitamin K.
2) Conversion of bile pigments & bile acids.
3) Absorption of nutrients & breakdown of products.
4) Antagonism to microbial pathogens, through bacterial
interference.
5) Anaerobic flora of the colon (B.fragilis, clostridia,
Peptostreptococci), plays a main role in abscess formation
after perforation of the bowel. Prevotella spp. cause
abscess formation in the pelvis of female genital organs.
Antimicrobial drugs taken orally suppress the drug-
susceptible flora, replaced by drug-resistant ones
(staphylococci, enterococci, enterbacters, Pseudomonas,
C.difficile & yeasts).By feeding large quantities of
Lactobacillus acidophilus → partial suppression of gut
microbial flora.
N.F of the Urethra:
Anterior urethra of both sexes contains small No. of the
same organisms found on the skin & perineum → appear
in normal urine (10
2
-10
4
org./ml).
N.F of the Vagina:
Soon after birth: aerobic lactobacilli appear & persist as
long as pH acid for several weeks, when pH become
neutral (until puberty) → mixed flora of cocci & bacilli.
At puberty: Aerobic & anaerobic lactobacilli
(maintenance of acidic pH by production of acid from
carbohydrates; glycogen) → preventing the establishment
harmful microorganisms in the vagina.
If lactobacilli suppressed (by drugs) → increase No. other
bacteria & yeasts----- irritation & inflammation.
After menopause: Lactobacilli again increase with a
mixed flora (group B streptococci, peptostreptococci,
clostridia, Prevotella, Gardnerella vaginalis, Ureaplasma
urealyticum, sometimes lesteria or Mobiluncus spp.,
E.coli or coliform).
The cervical mucus has antimicrobial activity & lysozymes
In some women, vagina contains a heavy flora resembling
that of perineum & perianal area → predisposing factor in
recurrent UTIs & at delivery may infect the newborn
(e.g., group B streptococci).
N.F Of The Eye (conjunctiva):
Diphtheroids (Corynebacterium xerosis), S. epidermidis,
neisseriae, nonhemolytic streptococci, G- bacilli
(Haemophilus, Moraxella spp.).
Bacteroides fragilis,