Microbiology Ch 2 11-17
The Normal Microbiota
-normal microbiota defined as microorganisms found frequently on the body of healthy people
-many people have the “core” microbiota, and others have transient, meaning they come and
they go in sporadic waves
-disease of transient microbiota doesn’t begin without colonization first
-signs and symptoms of infection many times caused by body’s own inflammatory response
-in human body, most efficient microbe is one that rapidly and efficiently becomes 2 microbes
Typical Sites on the Body for Colonization
-Skin – most areas like groin and toes
-Respiratory tract – nose and oropharynx
-Digestive Tract – mouth and large intestine
-Urinary Tract – anterior parts of the urethra
-Genital System – vagina
-presence of organisms in blood, CSF, synovial fluid, and deep tissues can be pathogenic
-feces are about 1/3 bacteria by weight
How do Microorganisms Persist in the Body – must resist host defense mechanisms, such as
antibodies, antibacterial enzymes, etc…
-in smokers, ciliary clearance mechanisms in respiratory tree are diminished, allowing microbes
to colonize and cause aspiration pneumonia
-colonization involves both microbial and host factors
-fibronectin, an adhesive protein coating mucosal surfaces of epithelial cells attracts
gram-positive bacteria, and is important in determining nature of bacterial microbiota in
mouth and pharynx
-low levels of fibronectin allow gram-negative organisms to displace gram positives
Anticolonizing Property of Host
Sweep microbes away by liquid currents
Kill microbes with host phagocytes
Starve microbes for lack of needed nutrients
Inhibit growth by secreting antimicrobial
factors such as cationic peptides
How Bacteria Overcome Property
Adhere to epithelial cells
1. avoid being taken up
2. kill the phagocyte
Derive needed nutrients from host cells
Modify surface molecules to avoid cationic
peptide binding
Site Preferences of Some Infectious Agents – microorganisms seem to cause disease only in
specific organs (hepatitis viruses in liver, encephalitis and rabies  brain)
-some bacteria have strong preferences, whereas some (staphylococcus) may infect any site of
body; preferences are called tissue tropism
-depends on tissue-specific cellular properties, temperature, other properties
-use gonococcus for example, causes infection of urethra, but can affect throat, rectum, or eyes
-where lesion occurs depends on site of entry
-pharyngeal and rectal gonorrhea can result from nonvaginal intercourse
-ophthalmia is caused by infection of eyes in neonate as baby passes through vagina of
infected mother
-viruses have tropism for specific cells that have receptors for attachment sites, such as
glycoproteins for influenza, or HIV receptors on lymphocytes and macrophages
-temperature of organ can determine location of infection, such as common cold in nose
Common Source of Infection – some normal, constituent microorganisms find themselves in
unaccustomed sites of body and can cause disease
-Bacteroides are anaerobic bacteria in intestines and help digest complex
polysaccharides; but produce abscesses if penetrating into deeper tissues
-no organism is intrinsically benign or pathogenic, and under right circumstance, can cause
disease; some more readily than others
Immune Stimulation – normal repertoire of antibodies reflects antigenic stimulation by normal
microbiota
-in general, we don’t have high Ab titers to bacteria, viruses, and fungi in our bodies, but they do
serve as defense mechanism
-antibodies against bacteria are often of the IgA class and secreted through mucous membranes
-antibodies of antigenic microbiota can cross-react with normal tissue components, such as
those of ABO blood groups, where the anti-A and anti-B antibodies have an unknown source in
newborn
-some bacteria in the intestine have antigens that cross-react with both A and B blood
antigens and our body makes antibodies against them
-some cross-reactivity can be protective, such as antibodies to bacteria in bowel
normally can cross react with polysaccharide capsule of meningitis-producing
meningococci
-microbiota also help develop our immune system by helping T helper cells, and Th17 cells
differentiate and help regulate mucosal immunity
Keeping out Invaders – normal microbiota can help keep out pathogens, called previous
occupancy, especially on epithelial surfaces
-some normal microbiota produce substances to inhibit newcomers like antibiotics or lethal
proteins called bacteriocins
-some enteric pathogens induce inflammation to kill off normal microbiota so they can thrive
-when normal microbiota are wiped out with antibiotics, both exogenous and endogenous
-patients treated with antibiotics effective in the gut may suffer diarrhea caused by toxins
produced from overgrowth of Clostridium difficile, which can cause pseudomembranous colitis
Role in Human Nutrition and Metabolism – normal microbiota plays a role in nutrition and
metabolism in the intestine
-digestion of complex polysaccharides is carried out by microbiota, and E. coli and bacteroides
synthesize vitamin K and are helpful in clotting
-liver synthesizes steroids and bile salts in the form of glucuronides or sulfate, but cannot be
absorbed that way; intestinal microbiota help the absorption by deconjugating the compounds
-genomes of microbiota collected through the microbiome
Good and Bad Conversion of Ingested Compounds – some compounds we ingest can be
transformed by microbiota into harmful compounds such as carcinogens in the small intestine
-artificial sweetener cyclamate (cyclohexamine sulfate) is converted to active bladder
carcinogen cyclohexamine by bacterial sulfatases
-some microbiota also detoxify certain carcinogens
-some carcinogenic heterocyclic amines in cooked meat are made less toxic by microbiota
Members of the Normal Microbiota – most normal microbiota are bacteria, but some are
viruses, fungi, protozoa, and maybe worms, but smaller numbers in than bacteria in normal
people
-E. coli was believed to be the principal member of fecal microbiota because most members of
normal bacterial microbiota are anaerobes and do not grow on media incubated in normal air
Site
Skin
Oropharynx
Gram-Positive
Cocci
Rods
Corynebacterium,
Staphylococcus Propionibacterium
acnes
α-Hemolytic
streptococci,
Corynebacterium
Micrococcus
Gram-Negative
Cocci
Rods
Other
---
Enteric
bacilli
---
Neisseria
Haemophlus
Spirochetes
Large
Intestine
Streptococcus
(enterococci)
Lactobacillus,
Clostridium
---
Vagina
Streptococcus
Lactobacillus
---
Bacteroides,
enteric
bacilli
Bacteroides
--Mycoplasma