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Microbiology

CHAPTER 4: PROKARYOTIC DIVERSITY

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Question: How would the world change if all the prokaryotes disappeared? Discuss.

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Question: How would the world change if all life on earth except prokaryotes disappeared? Discuss.

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Ecological Associations Among Microorganisms�

Microbial Associations

Symbiotic

Organisms live in close

nutritional relationships;

required by one or both members.

Organisms are free-living;

relationships not required

for survival

.

Mutualism

Obligatory,

dependent;

both members

benefit.

Commensalism

The commensal

benefits;

other member

not harmed.

Parasitism

Parasite is

dependent

and benefits;

host harmed.

Synergism

Members

cooperate

and share

nutrients.

Antagonism

Some members

are inhibited

or destroyed

by others.

Nonsymbiotic

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Ecological Associations

Symbiotic: two organisms live together in a close partnership

Mutualism – obligatory, dependent; both members benefit
Commensalism – commensal member benefits, other member neither harmed nor benefited
Parasitism – parasite is dependent and benefits; host is harmed

Non-symbiotic: organisms are free-living; relationships not required for survival

Synergism – members cooperate to produce a result that none of them could do alone
Antagonism – actions of one organism affect the success or survival of others in the same community (competition)

Antibiosis

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Interrelationships Between Microbes and Humans

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Human body is a rich habitat for symbiotic bacteria, fungi, and a few protozoa - normal microbial flora

Commensal, parasitic, and synergistic relationships

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Microbial Biofilms

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Biofilms result when organisms attach to a substrate by some form of extracellular matrix that binds them together in complex organized layers

Dominate the structure of most natural environments on earth

Communicate and cooperate in the formation and function of biofilms – quorum sensing

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Biofilm Formation and Quorum Sensing

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Quorum Sensing

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The Cell Envelope

External covering outside the cytoplasm
Composed of two basic layers:

Cell wall and cell membrane

Maintains cell integrity
Two different groups of bacteria demonstrated by Gram stain:

Gram-positive bacteria: thick cell wall composed primarily of peptidoglycan and cell membrane
Gram-negative bacteria: outer cell membrane, thin peptidoglycan layer, and cell membrane

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Structure of Cell Walls

Determines cell shape, prevents lysis due to changing osmotic pressures
Peptidoglycan is the primary component:

Unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments

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Peptidoglycan layer

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Gram-Positive Cell Wall

20-80 nm thick peptidoglycan

Includes teichoic acid and lipoteichoic acid: function in cell wall maintenance and enlargement during cell division; move cations across the cell envelope; stimulate a specific immune response

Some cells have a periplasmic space, between the cell membrane and cell wall

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Gram-Negative Cell Wall

Inner and outer membranes and periplasmic space between them contains a thin peptidoglycan layer
Outer membrane contains lipopolysaccharides (LPS)

Lipid portion (endotoxin) may become toxic when released during infections
May function as receptors and blocking immune response
Contain porin proteins in upper layer – regulate molecules entering and leaving cell

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Structures of Gram-Positive and Gram-Negative Bacterial Cell Walls

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Comparison of Gram-Positive and Gram-Negative Cell Walls

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Comparison of Gram-Positive and Gram-Negative Cell Walls
Characteristic	Gram-positive	Gram-negative
Gram reaction	stain dark violet	stain red
Peptidoglycan layer	Thick (multilayered)	Thin (single-layered)
Teichoic acids	Present in many	Absent
Periplasmic space	Absent	Present
Outer membrane	Absent	Present
Lipopolysaccharide (LPS)	Virtually none	High
Lipid & lipoprotein content	Low (acid-fast bacteria have lipids linked to peptidoglycan)	High (due to presence of outer membrane)
Flagellar structure	2 rings in basal body	4 rings in basal body
Toxins produced	Primarily exotoxins	Primarily endotoxins
Resistance to physical disruption	High	Low
Inhibition by basic dyes	High	Low
Susceptibility to anionic detergents	High	Low
Resistance to sodium azide	High	Low
Resistance to drying	High	Low

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Concept Check:

Which of the following is NOT found in the Gram-Negative cell wall structure?

Porins
Teichoic Acids
Periplasmic Space
Lipopolysaccharides
Peptidoglycan

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Gram stains

Distinguish between Gram-positive and Gram-negative bacteria on the basis of differential staining with a crystal violet-iodine complex and a safranin counterstain based on the composition of the cell walls.

Gram-positive - retain crystal violet and stain purple
Gram-negative - lose crystal violet and stain red from safranin counterstain

Important basis of bacterial classification and identification
Practical aid in diagnosing infection and guiding drug treatment

Fig: Gram –ve and +ve bacteria

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The Gram Stain�

Microscopic Appearance of Cell

Chemical Reaction in Cell

(very magnified view)

Step

1 Crystal

Violet

(primary

dye)

2 Gram’

siodine

(mordant)

3 Alcohol

(decolorizer)

4 Safranin

(red dye

counterstain)

Both cell walls stain with the dye.

Dye crystals

trapped in cell

Crystals remain

in cell.

Red dye

has no effect.

Gram (+)

Gram (–)

Gram (+)

Gram (–)

Outer wall is

weakened; cell

loses dye.

Red dye stains

the colorless cell.

No effect

of iodine

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So what?

Many antibiotics attack cell wall

We don’t have one!

Gram negative walls are protected by membrane = some antibiotics can’t get in
Gram negative bacteria release endotoxins = endotoxic shock!

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Vancomycin mode of action (gram positive cell)

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Nontypical Cell Walls

Some bacterial groups lack typical cell wall structure, i.e., Mycobacterium and Nocardia

Gram-positive cell wall structure with lipid mycolic acid (cord factor)

Pathogenicity and high degree of resistance to certain chemicals and dyes
Basis for acid-fast stain used for diagnosis of infections caused by these microorganisms

Some have no cell wall, i.e., Mycoplasma

Cell wall is stabilized by sterols
Pleomorphic

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Cell Membrane Structure

Phospholipid bilayer with embedded proteins – fluid mosaic model
Functions in:

Providing site for energy reactions, nutrient processing, and synthesis
Passage of nutrients into the cell and discharge of wastes
Cell membrane is selectively permeable

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4.2: Proteobacteria

The largest phylum of bacteria.

Divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Epsilonproteobacteria.

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4.2: Proteobacteria

Gram-negative

Purple bacteria and their relatives

Some are normal human microbiota

Some are pathogens

Some are environmental bacteria

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Alphaproteobacteria

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Alphaproteobacteria�Unifying factors:�Oligotrophic – low nutrient environment�Some are obligate intracellular parasites

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Rickettsia rickettsii�R. rickettsii cells inside tick hemolymph cells via CDC on Wikipedia

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Rocky Mountain Spotted Fever

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Rickettsia rickettsii is transmitted to humans through a tick bite and is the most common rickettsial infection in North America

First symptoms are fever, chills, headache and a spotted rash appears in days

Central nervous system can become involved and fatality rates are 20% if untreated

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The Rickettsia Genus

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Small obligate intracellular parasites

Gram-negative cell wall

Nonmotile pleomorphic rods or coccobacilli

Ticks, fleas, and lice are involved in their life cycle

Bacteria enter endothelial cells and cause necrosis of the vascular lining – vasculitis, vascular leakage, and thrombosis

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EPIDEMIC TYPHUS – R. PROWAZEKII CARRIED BY LICE; STARTS WITH A HIGH FEVER, CHILLS, HEADACHE, RASH; BRILL-ZINSSER IS A CHRONIC, RECURRENT FORM

ENDEMIC TYPHUS – R. TYPHI, HARBORED BY MICE AND RATS; OCCURS SPORADICALLY IN AREAS OF HIGH FLEA INFESTATION; MILDER SYMPTOMS

ROCKY MOUNTAIN SPOTTED FEVER (RMSF) – R. RICKETTSII ZOONOSIS CARRIED BY DOG AND WOOD TICKS (ROCKY MOUNTAIN WOOD TICK); MOST CASES IN SOUTHEAST AND ON EASTERN SEABOARD; DISTINCT SPOTTED RASH; MAY DAMAGE HEART & CNS

EHRLICHIA GENUS (NAMED FOR THE DR. EHRLICH WHO FIRST DESCRIBED THEM) CONTAINS 2 SPECIES OF RICKETTSIAS; TICK-BORNE BACTERIA CAUSE HUMAN MONOCYTIC & GRANULOCYTIC EHRLICHIOSIS

Fig: RMSF, Source: https://www.cdc.gov/rmsf/symptoms/index.html

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Coxiella burnetti

Causes Q fever

Intracellular parasite related to the Rickettsiaceae family

Harbored by awide assortment of vertebrates and arthropods

Infectious material includes urine, feces, milk, and droplets

Usually inhaled causing pneumonitis, fever, hepatitis

Tetracycline treatment

Vaccine available

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The Bartonella Genus

Small gram-negative, fastidious, related to the Rickettsiaceae family

Pathogens:

Bartonella quintana - Trench fever, spread by lice

Bartonella henselae - Cat-scratch disease, a lymphatic infection associated with a clawing injury by cats

More severe bacillary angiomatosus can result from infection in AIDS patients

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The Chlamydiaceae Family

Small, gram-negative, obligate intracellular parasites
Alternates between:

Elementary body – small metabolically inactive, extracellular, infectious form released by the infected host
Reticulate body – noninfectious, actively dividing form, grows within host cell vacuoles

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Chlamydia trachomatis

Human reservoir and 2 strains can infect humans:

Trachoma – attacks the mucous membranes of the eyes, genitourinary tract, and lungs

Ocular trachoma – severe infection, deforms eyelid and cornea, may cause blindness
Inclusion conjunctivitis – occurs as baby passes through birth canal; prevented by prophylaxis
STD – second most prevalent STD; urethritis, cervicitis, salpingitis (PID), infertility, scarring

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Q: When they were first discovered, Rickettsia were originally thought to be a virus. Why?

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Betaproteobacteria

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Betaproteobacteria�Unifying factors:�Eutrophs – need lots of nutrients�Some are normal intestinal flora; some are pathogenic

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Family Neisseriaceae

Gram-negative cocci

Residents of mucous membranes of warm-blooded animals

Genera include Neisseria, Branhamella, Moraxella

2 primary human pathogens:

Neisseria gonorrhoeae
Neisseria meningitidis

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Genus Neisseria

Gram-negative, bean-shaped, diplococci
None develop flagella or spores
Capsules on pathogens
Pili
Strict parasites, do not survive long outside of the host
Aerobic or microaerophilic
Produce catalase and cytochrome oxidase
Pathogenic species require enriched complex media

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Neisseria gonorrhoeae: The Gonococcus

Causes gonorrhea, an STD

Virulence factors:

Fimbriae, other surface molecules for attachment; slows phagocytosis
IgA protease – cleaves secretory IgA antibody

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Epidemiology and Pathology

Strictly a human infection

In top 5 STDs

Infectious dose 100-1,000

Does not survive more than 1-2 hours on fomites

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Gonorrhea

With penis – urethritis, yellowish discharge, scarring, and infertility

10% of persons with penises are asymptomatic

Normal

Gonorrhea

Urinary bladder

Ureter

Seminal vesicle

Prostate gland

Vas deferens

Urethra

Penis

Epididymis

Testis

Site of infection

Urethral opening

Scar tissue

(a)

(b)

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Gonorrhea

With vulva – vaginitis, urethritis, salpingitis (inflammation of the fallopian tubes) (PID-Pelvic inflammatory disease) mixed anaerobic abdominal infection, common cause of sterility and ectopic tubal pregnancies

50% of women are asymptomatic

Normal

Fallopian tube

Ectopic (tubal) pregnancy

Scar tissue

Anaerobic infection

Peritoneum

Uterus

Cervix

Fimbriae

Ovary

Gonorrhea

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Gonorrhea in Newborns

Infected as they pass through birth canal
Eye inflammation, blindness
Prevented by prophylaxis immediately after birth

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Neisseria meningitidis: The Meningococcus

Virulence factors:

Capsule
Adhesive fimbriae
IgA protease
Endotoxin

12 strains; serotypes A, B, C cause most cases

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Epidemiology and Pathogenesis

Prevalent cause of meningitis; sporadic or epidemic

Human reservoir – nasopharynx; 3-30% of adult population; higher in institutional settings

High risk individuals are those living in close quarters, children 6 months-3 years, children and young adults 10-20 years

Disease begins when bacteria enter bloodstream, cross the blood-brain barrier, permeate the meninges, and grow in the cerebrospinal fluid

Very rapid onset; neurological symptoms; endotoxin causes hemorrhage and shock; can be fatal

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Gammaproteobacteria

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Gammaproteobacteria�Very diverse group

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Vibrio Cholera

Comma-shaped bacteria is ingested in food or water
El Tor biotype: survives longer, more infectious
Infects mucous barrier of small intestine, noninvasive
Releases cholera toxin that causes electrolyte and water loss through secretory diarrhea, “rice water stool”; resulting dehydration leads to muscle, circulatory, and neurological symptoms
Rehydrate and treat with Tetracycline

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AB Toxins�Public domain image by Jawahar Swaminathan via Wikipedia

Two-component (A and B) toxins from some pathogenic bacteria
A component = ENZYME
B component = BINDING

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Cholera Toxin Activity

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Impact of Cholera

7 cholera pandemics since 1817
El Tor pandemic is ongoing
Spread through inadequate sewage treatment
Can be deadly if untreated
2.9 million global cases/year
95,000 global deaths/year

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Other Vibrio Pathogens

Salt-tolerant inhabitants of coastal waters, associate with marine invertebrates

Vibrio parahaemolyticus – gastroenteritis from raw seafood; symptoms similar to cholera

Vibrio vulnificus – gastroenteritis from raw oysters; serious complications in persons with diabetes or liver disease

Treatment – fluid and electrolyte replacement; occasionally antimicrobials

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Deltaproteobacteria

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Deltaproteobacteria�Unifying factors:�Very small group�Some use sulphate�Most are not pathogenic (Exception: some cause periodontal disease)

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Gliding and Fruiting Bacteria

Gram-negative
Glide over moist surfaces
E.g. myxobacteria, Myxococcus xanthus, etc.

Gliding, swarming, self-organizing predator!

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Myxococcus xanthus predatory swarming

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Epsilonproteobacteria

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Epsilonproteobacteria�Unifying factors:�Smallest class�Microaerophilic

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The Campylobacter Genus

Slender, curved, or spiral bacilli, often S-shaped or gull-winged pairs
Polar flagella
Common residents of the intestinal tract, genitourinary tract, the oral cavity of birds and mammals
Most important:

Campylobacter jejuni
Campylobacter fetus

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Campylobacter jejuni

Important cause of bacterial gastroenteritis

Transmitted by beverages and food

Undercooked poultry

Reach mucosa at the last segment of small intestine near colon; adhere, burrow through mucus and multiply

Heat-labile enterotoxin CJT stimulates a secretory diarrhea like that of cholera

Symptoms of headache, fever, abdominal pain, bloody or watery diarrhea

Treatment with rehydration and electrolyte balance therapy

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Helicobacter pylori

A gastric pathogen

Curved cells discovered in 1979 in stomach biopsied specimens

Causes 90% of stomach and duodenal ulcers; apparent cofactor in stomach cancer

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Helicobacter pylori

People with type O blood have a 1.5-2X higher rate of ulcers
Potent urease producer, which converts urea into ammonium and bicarbonate that neutralize the acidic environment of gastric epithelia

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Our beliefs about the causes of ulcers have changed over time.

Image from Cartoonstock, used via Fair Use Clause

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Helicobacter pylori

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Sourcce:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1246068/

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