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Microbial interactions with plants/plant environment

• Roots: moisture and nutrients are more homogeneous than leaves and tops-lots of microbial activity

• Rhizospehere and rhizoplane- zone of activity and micocolonies formation due to secretion of AA, sugars, dead cells, vitamins

Agrobacterium and crown gall disease

• Plant pathogen• Causes tumors in various plants• A. tumefaciens and A. rhizogenes• Uncontrolled growth-tumor• Tumor formation continues in the

absence of Agrobacterium- Ti plasmid

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Microbe-Human Interactions Infection and DiseaseAn introduction & 3 case studies

1. Clostridium botulinum 2. Shigella flexneri 3. Mycobacterium tuberculosis

CENTRAL THEME

Pathogens strive to multiply and invade the host’s tissues.Hosts strive to block the invasion and to destroy the pathogen.

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Important Terms

normal floraNormal microbiota

uses a situation to gain advantage or powerOpportunist

an organism living in close association with another without being totally dependent upon it

Commensal

an animal or plant on which or in which another organism lives Host

a substance that injures a living thing upon contact or absorption, typically by interacting with biological molecules such as enzymes and receptors

Toxin

components of an organism that determine its capacity to cause disease but are not required for its viability per se.

Virulence factor

pathos (disease) genesis (development)Pathogenesis

relative ability to do damage to the hostVirulence/ Pathogenicity

Abnormal condition of body or mind that leads to discomfort, dysfunction or distress

Disease

Important Terms (continued)

Host defenses effective against most bacteria and are always present; include physical barriers, complement, phagocyticcells and washing action of fluids

Non-specific defenses system

Host defenses produced in response to invasion by specific bacteria or other infectious agents; includes antibodies, T cells and activated macrophages

Specific defense system

Number of bacteria or amount of toxin required to kill 50% of the animals tested; measure of lethality

LD50

Number of microorganisms required to cause infection in 50% of experimentally infected individuals; measure of infectivity

ID50

detrimental colonization of a host organism by a foreign speciesInfection

when a life form moves into an area where their kind is sparse or not yet existing

Colonization

a bacterium that retains the violet stain used in Gram's method;cell wall consists chiefly of peptidoglycan

Gram positive bacterium

a bacterium that does not retain the violet stain used in Gram'smethod; cell wall contains little peptidoglycan

Gram negative bacterium

lipopolysaccharide, forms outer coat of a gram negative bacteria

LPS

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Salmonella

Oxygen radicalsIntracellular life

Virulence Factors

• Extracellular proteins produced involved in maintenance and establishment of disease

• Hyaluronidase: breaks host cells apart-spreading of pathogen

• Other enzymes: proteases, nucleases, lipases, collagenase

• Fibrin clots: isolating mechanism- dissolved by fibrinolytic enzymes

• Some pathogens promote clots to protect infection site-coagulase- pimples

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Exotoxins• Proteins released extracellularly by the

pathogen• Translocate to distant parts and cause

damage• Cytolytic toxins- cell costituents• Assay for exotoxins: hemolysis• Substrates for hemolysins-

phospholipids/sterols/white blood cells• Usually come in combination of 2 units

Endotoxins• Toxins that are bound to the cell and are

released upon cell lysis• LPS produced by G(-) Bacteria- toxic• Effects: fever due to release of endogenous

pyrogens, affect temperature control by the brain

• Diarrhea, inflammation, low counts of white blood cells

• Death- hemorrhagic shock • Low toxicity

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Never underestimate the power of bacteria….

Bacteria3.5 billion years ago

Earth 4.5 billion years old

Today

Humans 195,000 years ago

Why do bacteria colonize humans?

Time line

All bacteria need nutrients & most need heat to allow them to multiply

Humans are the proverbial “free lunch”

human-microbe interactions• Beneficial and harmful microbes• Everyday there is exposure to lots of

microbes from the environment• Normal flora• Dynamic interaction• Infection: establishment of pathogen in a

host• Host-parasite interactions

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Getting the balance right

Beneficial bacteria are necessary to keep pathogenic bacteria in check

Bacteria interfere with bodily functions

Host does not benefit from the bacterial:host interaction eg. nutrient availability

Bacteria compete with the host for nutrients

Too fewToo many

Most bacteria are neutral or beneficial to humans e.g. and normal microbiotacommensalsHowever without our defense systems these bacteria may become problematic to humans i.e immuno-compromised patients

How do humans benefit from bacterial colonization?

Host-parasite interactions• Entrance: mucous membranes• Found in areas where there is contact with exterior: oral, IT, RT.• Cells covered with glycoprotein• Bacteria associate with surface• Specific interactions may result in invasion of deeper tissues

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Flora of the skin• Microbes associated with

sweat glands (apocrine)• Warm, humid places – high

populations • Armpit odor-bacterial activity• Secretions: urea, aa, salts and

lactic acid and lipids• Transient and resident

populations• G(+) in high populations

adapted to dry environments• G(-) low numbers due to

inoculation via fecal sources• If immuno system is weak: any

microbe may invade• Abnormal flora: Candida

Low water contentLow pH

Oral cavity• Heterogeneous habitats• Saliva: nutrient source• Food particles and epithelial debris• Also antibacterials lysozyme and

lactoperoxidase (generate oxygen radicals, lethal, similar to hydrogen peroxide)

• Tooth: mineral matrix and living tissue

• Promote anaerobic microbes adapted for growth on tooth surfaces

• Coated with glycoproteins- provides attachment surface

• Mostly Streptococcus sanguis, S. sobrinus, S. mutans, and S. mitisgrowth- dental plaque

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• Plaque accumulation- caries result• Acid accumulation• Site: crevices- accumulation of food• Dogs teeth resistant to accumulation

of food• Diet sugars: favor acid production and

dental decay- fermentation and production of lactic acid dissolves the enamel, gained access to matrix by production of proteolytic enzymes

• Fluoride-resistant enamel• S. mutans produces dextransucrase,

which degrades sucrose (sugar)• Developed countries sugar in diet• Tanzania almost no cavities

GI tract• Stomach pH 2, barrier for entry

of bacteria, low counts• Helicobacter pylori- ulcers• Small intestine: duodenum,

jejunum and ileum• Large intestine: large numbers

107/gm (facultative aerobes)• Obligate anaerobes: 1010-1011

• Bacteroides- dominant (meat)• Lactic acid and coliforms (veg)• Vitamin B12 and K, modified

steroids• Methanogens!!• Chemostat• Antibiotics treatment

Acidic

107-105

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Flora of the body• Different mucous membranes

have different populations• Pathogens do not become

pathogens because of competition with the flora

• Respiratory tract: upper and lower

• Upper: residence on membranesMost microbes are trapped in nasal

passage: Staph, Strep• Lower: essentially sterileParticles 10 microns in size may

reach the lungs

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Urogenital tract• Bladders are usually sterile• Urethra is lined with mucous

membranesColonized by facultative

anaerobes G(-) rods and cocciProteus, Escherichia may

become pathogen (opportunistic)-changes in the environment

• Vagina: acidic pH, glycogen Lactobacillus generates acid and

lowers the pHTorulopsus, Candida• Prior to puberty and

menopause: alkaline, no glycogen- Staph, Strep

Bacterial pathogens

Bacterial Pathogen = Bacterium that causes disease

2 types of bacterial pathogens

2. Opportunistic Pathogens- Capable of infecting only when host defenses are compromised

eg. Pseudomonas aeruginosaFound everywhere in the environment Can infect lungs of cystic fibrosis patientsAnd skin of burns patients

eg. Yersinia pestisBacterial agent that cause plague.

1. Primary Pathogens – Cause disease in healthy individuals.

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Human defenses

Human behaviorHand washing,Taking entire prescription of antibiotics

The body’s defenses2 levels of defense

Non-specific (the first line of defense)- always present i.Physical and Chemical barriers:Eg. Skin, epithelial cells, mucin, stomach acid, antimicrobial peptides

ii.Phagocytes, unactivated macrophages and Natural killer cells

Specific defenses (produced in response to invasion by specific infectious agent)Antibodies, cytotoxic T cells and activated macrophages

Strategies adopted by bacteria to colonize the host

e.g. Pseudomonas syringae – relate back to pathogensColonization and invasion of host surfacesPenetrating intact skinAcid tolerance (stomach and skin)Penetrating mucin layerResistance to antimicrobial peptides and antibioticsAdherencesIgA proteasesIron acquisition Invasion and intracellular residence

Evading complement, phagocytes and the antibody responseCapsulesStrategies for circumventing complement and phagocytesSurviving phagocytosisResistance to NOEvading the body antibody response

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Clostridium botulinum

The facts:Gram positive spore-forming bacterium

Anaerobic

Soil inhabitant, spores found on fecal-contaminated plants

Spores can be carried by bees –honey has trace amounts of toxin risky for small children

Bacteria produces a toxin which is entirely responsible for symptoms in humans= BOTOX

Mode of action and symptoms

Toxin is a multi-subunit protein complex

Blocks neurotransmitter (acetylcholine) release from peripheral nerve endings

Once toxin enters a nerve-ending intervention becomes useless

Within 4-36h this results in flaccid paralysis

Death occurs if nerve in hearts and lungs are affected

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What is the benefit of the toxin to Clostridium?

Botulinum toxin

Toxin is the major virulence factor

Is Clostridium botulinum truly a pathogen or simply a bacterium in the wrong place at the wrong time ?

Nicked toxin Binds neurons InternalizedPrevents acetylcholine release

Foodborne botulism occur most years and are usually caused by eating contaminated home-canned foods. Bacteria do not survive in the colon due to competition with natural microbiota.

Infant botulism – occur due to a underdeveloped microbiota in the gut of babies. Opportunistic anaerobe.

Wound botulism develops when deep wounds are contaminated with soil

-Average of 110 cases of botulism are reported each year in US. 25% are food-borne, 72% are infant botulism

-However, incidence of wound botulism is increasing with use of black-tar heroin, especially in California..

Diseases associated with Clostridium botulinum

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-Controlling muscle spasms, and crossed eyes

-Injections of BOTOX can control movement of limbs in cerebral palsy patients

-In 2002, FDA approved use of BOTOX in cosmetic surgery to reducewrinkles

Medical applications for BOTOX

Shigella spp.

♦ Symptoms: Diarrhea, fever, stomach cramps, may results in dehydration and death

♦ Population at most risk: Elderly and very young

♦ Classified as a category B pathogen

♦ The CDC requests that Shigella outbreaks are reported

♦ Gram negative rod shaped bacteria

♦ 4 species: S. sonnei; S. flexneri; S. boydii; S. dysenteriae

♦ Source of pathogen: Contaminated food and water

♦ Disease: Shigellosis, disease of lower intestine

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Epidemiology of Shigella infections

♦Shigella causes 163 million cases of diarrhea and dysentery per year 1 million deaths

Most cases occur in the developing world

Globally

300,000 cases occur every year in the U.S

Recent cases: Salsa led to 300 cases in CA, 2000Layer Dip led to 295 cases in CA / WA, 2000Tofu salad led to 3175 cases in MI, 1988

In USA

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2 major factors contribute to occurrence of Shigellosis

- Poor sanitation

- Lack of hygiene during food preparation, or person to person contact

Problems associated with Shigellosis

-Children under 5 yrs are most susceptible

-Antibiotic resistance is on the increase

Shigella enters the gastro-intestinal tract

Acid tolerance

Resistance to antimicrobial peptides and antibiotics

Has an exceptionally low ID50=100

Mechanisms used by Shigella to colonize the host

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Shigella invade the colonic epithelium

Mechanisms used by Shigella to colonize the host

Penetrates epithelial layer by using scavenger cells

Survives phagocytosis by macrophages and kills them

Invades and resides intracellularly preventing exposure to the hosts antibody’s and non-specific extracellular defenses

Inflammation causes the epithelium to become leaky allowing other bacteria to enter tissue

ActinActin--based movement allows intracellular and based movement allows intracellular and intercellular spreadintercellular spread

A single bacterial protein IcsA is absolutely required for intracellular movement

This protein is a major Shigella virulence factor

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Mycobacterium tuberculosis – the perfect pathogen

-Obligately aerobic

-Rod shaped bacterium

-Gram positive bacterium type cell wall

-High lipid content (up to 10%)

-Special stain required (acid fast stain)

Disease:Tuberculosis (TB); disease of the lungs

Reservoir:Humans and especially those with latent TB

The bacterium:

2 billion infected and 800,000 new cases per year

One of top 3 killers after HIV and malaria

A Historical perspective

-1800’s Epidemic in US, Europe annual death toll of 1% in major cities

-1950’s Discovery of a cure for TB, development of inexpensive tests to detectinfected people (skin test and chest X-ray) and laws introduced requiring

patients to complete treatment

-1970Number of TB cases low, anti-TB programs dismantled

-mid-1980’s to 1990’sNumber of cases increase. Likely reasons: immigration and number of HIV

patients increase; appearance of antibiotic resistant strains

M. tuberculosis ranks as the top bacterial pathogen in the world

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Symptoms

FeverCoughing ( with bloody sputum)Weight lossLoss of energyProgressive and irreversible lung destructionBacteria may escape the lungs and enter the bloodstreamCan infect any area of the body where there is sufficient oxygenSystemic form of the disease is normally fatal

Mode of spread

Aerosolized making it highly contagious e.g prisons and school bus

Transmission most likely after prolonged close contact, although contact doesn’t have to be intimate

M. tuberculosis can survive in un-activated macrophages but is killed by activated macrophages

Steps in the development of tuberculosisInhalation of bacteria

Bacteria reach lungs and enter macrophages

Bacteria reproduce in macrophages

Lesion/ Tuberculebegins to form

Lesion liquefies

Spread to blood, organs

Death

Bacteria coughed up in sputum

Activate macrophages

Bacteria cease to grow; lesion calcifies

Immune suppression

Reactivation

M. tuberculosisDead phagocytes

Phagocytes, T cells, and B cells trying to kill bacteria

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Mechanisms used by M. tuberculosis to colonize the hostIn Macrophage Enters macrophages in abnormal way and so bypasses normal killing mechanism of the macrophageReduce killing potential of the macrophageDecreases signals to other parts of the non-specific and specific defense systemsAntibodies produced are useless as the bacteria are multiplying inside phagocytes

In lungMacrophages are attracted to the area where the bacteria are growing, causing massive tissue damageA layer of T cells and macrophages form around the damaged tissue containing the bacteriaCalcification of these cells effectively wall-off the infection and prevents further spread = GRANULOMAS. Bacteria can survive in Granulonas for decades and may break out at a later stage in the patients life = REACTIVATION TBBacteria cause liquefaction of lung tissue. This renders aerosols facilitating transmission to new host.

Mycobacterium tuberculosis – the perfect pathogen

Slow killer

Highly contagious

Method of transmission through aerosols

Lie dormant in the host for long periods

Resistant to many aspects of the immune system

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SUMMARY OF BACTERIAL PATHOGENESIS

Bacterial pathogenesis is a battle between host and pathogen

Complex interplay between host’s defenses and bacterial virulence strategies

A wide variety of mechanisms can result in disease, toxins which have no known benefit to the bacteria or long term persistent bacterial infections as with tuberculosis

CENTRAL THEMES

Pathogens strive to multiply and invade the host’s tissues.

Hosts strive to block the invasion and to destroy the pathogen.

Frequently the hosts immune response to the bacterial infection is responsible for symptoms in the host