Pathogenic Mechanisms of Bacteria

1
Study Objectives What structural features of
bacteria differentiate them from eukaryotic
cells? What is the morphology of a bacterium,
and what structures are recognized? The student
must be able to describe differences between
Gram and Gram- cell walls and how this impacts
pathogenesis. An understanding of spore
formation and utility is required. One should be
able to specifically describe immune mechanisms
against bacteria, both at body surfaces and in
the blood. What molecules participate and how do
they work? What is a respiratory burst? How do
acute phase proteins contribute to defense? What
is the role of Ab? Conversely, one should be
able to describe mechanisms that bacteria have
evolved to avoid specific immune processes. You
are NOT required to know specific bacteria or
diseases. Terminology of host-pathogen
interactions is important. Also, what are
exotoxins, and how do they elicit disease? The
site of action for the antibiotics should be
understood, and subsequently how bacteria have
gained resistance to classes of antibiotics. The
only specific example required is that of
ß-lactam resistance. Finally, how do we
determine whether an isolate is resistant to an
antibiotic? What are Kirby-Bauer plates? How do
you determine a zone of inhibition?
2
Innate Immunity
opsonization complement neutralization
phagocytosis Ag processing Ag presentation MHC
APC
Humoral
B cells
Antibodies (Ab)
Adaptive Immunity
CMI
TH cells
TC cells
Macrophages
Cell killing
enhanced phagocytosis bacteriacidal
activity cytokine production
3
Eukaryotic versus prokaryotic cells

• Bacterial Structure
• Cells Eukaryotes Eubacteria
• Organelles Many Never
• Nucleus Nuclear membrane with DNA in direct
  contact with spindle fibers cytoplasm
• Size 10-50 microns 0.2-5 microns
• Composition
• Lipids Complex phospholipids Infrequent complex
  lipids
• and sphingolipids
• Sterols Always Mycoplasma only
• Ribosomes 80S 60S 40S subunits 70S 50S 30S
  subunits
• Cell wall Absent (rare examples) Peptidoglycan

4

• Genetic Organization
• Chromosomes Many (usually) One circular (usually)
  plasmids
• Ploidy Usually diploid Haploid
• Replication Approx. 18 hours Usually lt 30 minutes
• Histones/ Present Absent
• nucleosomes
• Transcription Separated temporally Coupled with
  translation
• and physically
• from translation
• Genes Intervening sequences Colinear rare
  examples of introns
• These differences provide an advantage to the
  pathogen, the host, and the practitioner.

5
Bacterial Cell Morphology

• Bacterial cells display a variety of sizes and
  shapes, even within a single pure culture.
  Because the refractive index of bacterial cells
  is near that of water, it is difficult to view
  them with an ordinary light microscope. Bacteria
  are easily observed following a number of simple,
  rapid stains.
• There are 4 basic shapes of bacteria
• spherical (cocci)
• rod-shaped (bacilli)
• curved
• spiral
• It is important to remember that there is no
  correlation between shape/size/Gram stain and
  pathogenesis. There are also no Gram-negative
  cocci of veterinary importance.

6
Coccus
Diplococcus
Streptococcus
Staphylococcus
7
Spirochetes
Borellia anserina in plasma from a chicken.
Adapted from Hagen and Bruner, 8th edition, p. 46.
8
Bacterial Morphology
9
Cell Wall The bacterial cell surface is composed
of two major layers, the cytoplasmic membrane and
the cell wall. The cytoplasmic membrane consists
of a lipid bilayer interspersed with proteins.
The cell wall usually contains more than one type
of layer. It is the composition of the cell wall
which distinguishes Gram from Gram- organisms.
10
The cell wall consists of a rigid layer of
peptidoglycan which gives the cell its shape and
maintains stuctural integrity. Peptidoglycan is
essential for bacterial survival. Gram bacteria
have a peptidoglycan layer which is much thicker
and more highly cross-linked than a Gram-
organism.
11
Peptidoglycan Peptidoglycan is a polysaccharide
backbone composed of alternating residues of
N-acetylglucosamine and N-acetylmuramic acid.
The peptidoglycan chains are cross-linked via
peptide bridges to increase cell wall rigidity in
Gram organisms.
12
Teichoic acid Restricted to Gram cell walls,
these are repeating cross-linked simple sugars.
The structures can be highly antigenic, esp. when
attached to bacterial proteins.
13
(No Transcript)
14
(No Transcript)
15
External Structures

• Bacteria can have any or none of three
  non-essential appendages flagella, fimbriae
  (pili), or capsules. Flagella are attached to
  the cell wall and are used only for non-Brownian
  locomotion. They are composed of glycoproteins
  and are thus highly immunogenic. They can
  therefore be used for serotyping and are called H
  antigens. Flagella can be 1) monotrichous,
  lophotrichous, or peritrichous. Moreover, their
  distribution or presence can change within a
  single bacterial isolate under various growth
  conditions.
• Fimbriae are smaller protein appendages and are
  usually present in higher number on a cell. They
  are not used for motility, but can participate in
  attachment to host cells. Specialized pili (sex
  pili) are involved in genetic transfer between
  cells. Fimbriae are often lost during culture in
  vitro.
• Capsules are the outermost layer and provide a
  "slime" covering for certain bacteria under
  certain growth conditions. Capsules are not
  easily viewed under Gram's stain, thus
  specialized stains (India ink or CuSO4) which are
  excluded by the capsule are used. Capsules can
  be important virulence factors because they can
  disrupt phagocytosis, mediate attachment to host
  mucous layers, prevent dessication of the cell,
  hide bacterial surface protein antigens, and
  inhibit lysis within phagolysosomes. A very
  loosely defined or cohesive caspule is sometimes
  called a glycocalyx.

16
(No Transcript)
17
Flagella
Dermatophilus congolensis showing flagella.
Adapted from Hagen and Bruner, 8th edition, p.
291.
Bacillus piliformis showing peritrichous
flagella. Adapted from Hagen and Bruner, 8th
edition, p. 212.
18
Capsules
Bacillus anthracis in a bovine spleen viewed by
India ink staining. Adapted from Hagen and
Bruner, 8th edition, p. 207.
19
Spores

• Spores are structures designed to promote long
  term, highly stable survival of an organism's
  genetic material. Spores are extremely difficult
  to kill, thus spore-forming bacteria can survive
  in hostile environments in the environment for
  many years. When conditions change to favor
  bacterial growth, the spores germinate. Only a
  relatively few species have the capacity for
  sporulation.
• A spore consists of a core which contains the
  bacterial DNA, a few enzymes, and Ca2. The core
  is surrounded by peptidoglycan and protein. Very
  little water remains within a spore to increase
  resistance. Spores can be killed chemically or
  by pressurized steam heat.
• The stages of sporulation are shown below.
  Only one spore/bacterial cell is formed, and not
  all bacteria within the culture will form a
  spore. The process can proceed fairly rapidly (lt
  1h) when necessary.

20
Endospore
21
Antibacterial Defenses

• There are 3 basic processes or components which
  prevent colonization by pathogenic bacteria
• SURFACE BARRIERS
• INNATE IMMUNITY
• ADAPTIVE IMMUNITY
• Animals usually first encounter bacteria at their
  surfaces which contact the environment. Physical
  barriers such as the skin and epithelial cell
  layers prevent bacteria from entering the host.
• The microorganisms that are encountered in daily
  life only occasionally cause disease. Most
  bacteria are detected and destroyed rapidly by
  defense mechanisms that are not antigen-specific.
  These are the components of the INNATE IMMUNE
  SYSTEM.
• Only if an infectious agent overcomes or breaches
  the innate immune system does ADAPTIVE or
  SPECIFIC IMMUNITY ensue. This system generates
  highly specific molecules and cells which
  interact only with elements of the invading
  organism. Memory is a key part of the adaptive
  immune system.

22
Immunity to Bacteria

• The ability of a host animal to successfully
  defend itself against invading organisms depends
  in part on the pathogenic mechanism of the
  bacterium, the site of infection, and the
  cellular location of the invading organism.
• Cell-Mediated Immunity Humoral
• Pathogen Vaccinia virus M. tuberculosis Cl.
  tetani
• Influenza virus M. leprae S. aureus
• Listeria Leishmania S. pneumoniae
• Toxoplasma gondii Pneumocystis Pneumocystis
• Location cytosol Macrophage Extracellular
• vesicles
• Effector T cell CTL TH1 TH2
• Ag Present Class I on Class II on Class II on
• infected cell infected mø specific B cell
• Action Cytolysis Mø activation B cell activation

23
Surface Defenses

• Non-specific
• Skin and epithelial barriers
• Resident microflora
• skin, large intestine, vaginal tract
• Low pH
• skin, stomach
• Flushing action
• lungs, urogenital tract, eyes, mouth
• Lysozyme, lactoferrin, defensins
• in milk, tears, mucous
• Phagocytic cells
• lungs, mammary glands
• Complement proteins
• Specific/Immunologic
• Tc
• IgA antibodies
• milk, mucosal linings

24
(No Transcript)
25
Innate Immunity

• Innate immune defenses are not specific to any
  single pathogen or antigen.
• Innate immunity is pre-existing.
• Innate immune responses are immediate and require
  no cellular differentiation or proliferation
• Most facets of the innate immune system are
  distributed throughout the body.
• Components of the innate immune system are the
  first molecules and cells which encounter
  invading organisms.

26
Phagocytosis

• The most important accessory cells in the innate
  defense against bacteria are PHAGOCYTES which
  engulf and destroy bacteria. Phagocytosis is the
  process of ingesting and destroying particles
  such as bacteria.
• The phagocytes mostly consist of MACROPHAGES and
  NEUTROPHILS.
• Many bacteria can be directly recognized,
  ingested, and destroyed by phagocytes.
• Phagocytes also have specific receptors which
  help in ingesting certain bacteria.
• Ingestion of microorganisms causes lysosomes to
  fuse with the phagosome. This process releases
  the contents of the lysosome to
• acidify the phagolysosome
• generate an oxidative burst which produces
• O2-, H2O2, NO, and halides (OCl-)
• release cationic peptides and defensins
• release proteases such as lysozyme
• activate competitor enzymes such as ferritin

The binding and phagocytosis of bacteria is
greatly enhanced by the process of OPSONIZATION.
This process puts molecules on the surface of the
bacterium. The macrophages and neutrophils have
receptors which recognize these molecules,
thereby enhancing the phagocytic process. The
receptors also signal the phagocytes to activate
the respiratory burst and other activities.
27
Phagocyte Anti-microbicidal Processes

• Degranulation and phagosome formation
• Acidification
• Cationic proteins
• Proteases
• Lactoferrin
• Lysozyme
• Acid hydrolases
• Myeloperoxidase
• Oxidative burst
• Hydrogen peroxide
• Superoxide
• Singlet oxygen
• Iodination
• Nitric oxide

28
Acute Phase Proteins

• The ACUTE PHASE RESPONSE is an indicator of
  inflammation. They are produced by hepatocytes
  and serve several important functions in
  antimicrobial responses
• several acute phase proteins bind to structures
  on the bacterium these structures activate
  COMPLEMENT.
• other acute phase proteins bind up free metal
  ions which inhibit bacterial growth.

29
Acute Phase Proteins and Complement

• Acute phase proteins can also serve to activate
  complement components, thereby enhancing
  phagocytosis.

30
Macrophage Receptors for Phagocytosis

• Macrophages in particular have many receptors
  that facilitate uptake of bacteria
• Complement receptors
• Fc receptors
• Mannose receptors
• LPS receptors
• Scavenger receptors

31
The Complement System

• A series of heat labile proteins which function
  in a cascade to
• kill bacteria
• opsonize bacteria
• recruit inflammatory cells
• Complement proteins are present in inactive forms
  in the serum and are triggered via either of 2
  mechanisms
• Classical Pathway
• activated by IgG or IgM
• Alternative Pathway
• activated by bacterial cell surfaces, acute phase
  reactants, or IgA

32
Complement Opsonization
33
Inflammation

• Phagocytes, primarily macrophages, when activated
  by ingestion of bacteria, release factors which
  increase vascular permeability and therefore
  allow more immune cells and factors to reach the
  site of infection.

34
Specific Immune Defenses

• Specific immune defenses are directed only
  against the invading pathogen.
• Specific immune defenses work by
• neutralizing antibodies
• activation of complement
• antibody opsonization to
• enhance phagocytosis
• activation of macrophages

35
Role for Antibody in Antimicrobial Defense
36
Host Defenses Against Bacterial Pathogens

• 1. Body Surfaces.
• Skin and mucosal surfaces -- the body's first
  line of defense. Nonspecific constitutive and
  specific induced defenses of the skin and mucosal
  surfaces are critical for the prevention of
  bacterial adherence and colonization.
• Defenses of skin and mucosal surfaces
• (adapted from Salyers and Whitt, p. 7)
• Site Defense Function
• Skin Dry, acidic conditions Limit bacterial
  growth
• low temperature
• sloughing cells Remove bacteria
• resident microflora Compete for colonization

37

• Hair follicles, lysozyme, toxic
  lipids Bactericidal
• sweat glands
• Dermis SALT Bactericidal, immune
• functions
• Mucosal surface Mucin Physical barrier
• Mucin layer Lysozyme Bactericidal
• sIgA Prevents attachment, traps
• bacteria, immune function
• Lactoferrin Bacteriostatic
• Lactoperoxidase Bactericidal
• Mucus Sloughing cells Remove bacteria
• membranes Tight junctions Physical barrier
• Beneath mucosa MALT/GALT sIgA, immune function

38

• 2. Defenses of Tissue and Blood.
• Defense Location Function
• Transferrin Blood/tissue Limits iron availability
• PMNs Blood/tissue Phagoctyosis/immune
  function
• Monocytes Blood Cytokines/immune function
• Macrophages Tissue Phagoctyosis/Ag
  presentation
• Complement Blood/tissue Phagocytosis/ opsoniza
  tion/
• bactericidal

39

• Mannose Produced by liver Complement activation
• binding protein
• T cells Blood Cellular and humoral immune
  functions
• B Cells Blood Humoral immune function
  (antibody production)
• Antibodies Blood Bacterial opsonization,
  complement activation, toxin neutralization
• (adapted from Salyers and Whitt, p. 17)

40
Surface Protective Systems
St. Paul Pioneer Press, April 19,1998
41
Bacterial PathogenesisTerminology

• Symbiosis Denotes a situation when an organism
  spends all or a portion of its life associated
  with an organism of a different species.
  Usually, symbiosis "intimate living together"
  between different species.
• Symbiosis is dynamic -- may change from
  beneficial to harmful as environment changes.
• Mutualism -- benefits both organisms.
• Commensalism -- the commensal benefits, the host
  is neither helped nor adversely effected.
• Parasitism -- the parasite either harms or lives
  at the expense of the host. May be of different
  types (external, internal, extracellular, or
  intracellular).

42

• Pathogens disease causing microorganisms.
  Often there is cooperation amongst pathogens --
  synergism and opportunists.
• Pathogenesis the physiological processes
  involved in the generation of clinical signs of
  disease the means by which a pathogen causes
  illness (i.e. through the action of direct damage
  to the host cells, or via endotoxins or
  exotoxins).
• Pathogenicity the capacity of a microbe to
  cause disease.
• Virulence -- ability of a microbe to cause
  disease efficiently (high virulence) or
  inefficiently (low virulence) -- also refers to
  the degree of pathogenicity.
• Virulence factor -- component of a pathogen
  that contributes to its disease-producing
  potential, e.g. toxins and surface molecules.

43
Basic Mechanisms of Bacterial Virulence and
Pathogenicity

• To induce disease, a pathogen must be able to
• Have access to a host (direct contact,
  fomites, vectors, etc.)
• Adhere to, and colonize or invade the host
• Successfully evade host defense mechanisms
• Replicate within or on the host whilst
  continuing to evade host defenses
• Be directly or indirectly capable of causing
  damage to host cells or tissues

44

• Virulence factors -- enable bacteria to overcome
  the hosts defenses and cause disease.
• Broadly speaking, virulence factors can be
  classified into two major categories
• Factors that promote bacterial colonization
  and invasion
• Factors that cause damage to the host.

45
I. Virulence Factors that Promote Colonization
and Survival

• Virulence factor Function
• Fimbriae (pili) Adherence to mucosal surfaces
• Nonfimbrial adhesins Binding to host cells
• Invasins Bacterial invasion of host cells
  (often mediated by triggering of host cell
  actin rearrangment)
• Motility and chemotaxis Reaching target
  tissue/cells
• IgA proteases Prevent sIgA action

46

• Siderophores, siderophore- Iron acquisition
• binding surface proteins
• Capsule Prevents phagocytosis resists
  complement action
• C5a peptidase prevents complement action
• Toxins Kill phagocytes promote
  colonization and dissemination
• Variation in surface antigens Evasion of antibody
  response
• Spores Environmental stability/persistance
• Anti-oxidant enzymes Prevent effects of oxidative
  burst
• Escape from the phagolysosome Prevents
  degradation of the bacterium
• (adapted from Salyers and Whitt, p. 31)

47
Anti-oxidant enzymes
Evading intracellular destruction
2 O2- 2H -----------------gt H2O2 O2
Superoxide dismutase
catalase
H2O O2
Many bacteria can resist killing within the
phagolysosome via several mechanisms.
Other bacteria can deactivate the products
generated by phagocytes that normally help kill
bacteria
48

• Anthrax
• Spores are important virulence factors for
  anthrax in that they promote survival in the
  environment. If you block spore formation,
  little virulence is observed.
• Ingestion/inhalation of spores
• spores germinate into vegetative bacilli
• organism fluorishes on mucosa
• septicemia
• rapid disease characterized by respiratory
  distress, drooling, stupor, convulsions and
  collapse
• death by asphyxiation and organ collapse

Transfer of B. anthracis spores by blowflies to
vegetation.
Grazing animals then ingest the spores,
permitting the bacterium to vegetate and produce
toxins that quickly kill the animal.
49
II. Virulence Factors that Damage the Host

• Bacterial toxins that are excreted are termed
  exotoxins, a term intended to differentiate these
  molecules from endotoxin, a component of the
  gram-negative outer-membrane.

• Exotoxins contribute to disease by several
  mechanisms.
• Exotoxins produced by bacteria growing in a
  wound or abscess cause local tissue damage
  enabling the bacteria to further invade host
  tissue and spread.
• Ingestion of preformed toxins -- the symptoms and
  clinical signs result from the action of
  preformed toxins rather than by bacterial
  infection.
• Bacteria colonize mucosal surfaces and produce
  toxins that either act locally or enter the
  bloodstream and cause damage at a distant site

50

• Exotoxin Endotoxin
• Excreted by living cells Part of outer membrane
  released during vegetative growth or cell
  death
• Polypeptides Lipid portion (lipid A) of
  lipopolysaccharide (LPS)
• Usually denatured at Relatively thermostable (can
  temperatures above 60C withstand autoclaving)
• Usually antigenic Lipid A is not antigenic
• Can be converted to toxoid Cannot be converted to
  toxoid
• Effects are usually cell/tissue/ Effects are
  generalized
• organ specific
• Relatively small lethal dose Considerably large
  lethal dose - animals differ in
  susceptibility

51

• Exotoxin Structure
• Three major categories based on molecular
  structure and activities.
• 1. A-B toxins. Have two components the "A"
  component -- exhibits toxic or enzymatic
  activity, and the "B" component -- binds the
  target host cell receptor. Eg. tetanus and
  shiga-toxins.
• 2. Membrane disrupting toxins. Act by forming
  pores (e.g. staphylococcal alpha toxin) or
  enzymatically disrupting the host cell plasma
  membranes (e.g. Clostridium perfingens a toxin)
• 3. Superantigens. Lack the classic A-B toxin
  structure and act by indiscriminately stimulating
  a large fraction of T cell populations and
  causing them to release abnormally high levels of
  cytokines that result in a variety of symptoms
  (e.g. staphylococcal enterotoxins).

52

• Cl. chauvoei
• Blackleg in ruminants
• Exposure is via ingestion of spores or through
  puncture wounds or during teething.
• Spores germinate and the organism replicates.
• The organism moves through the lymph to the
  blood.
• Organism localizes in muscle and liver.
• Cl. chauvoei remains dormant in tissue until
  trauma occurs.
• Replication and production of toxins.
• Sudden onset of fever and tissue necrosis with
  swelling and gas.
• Death can occur in 1-3 days due to toxemia. The
  infection can self-cure, with the tissue damage
  repairing itself.

• Toxins
• alpha toxin lethal and necrotizing
• beta toxin DNase activity
• gamma toxin hyaluronidase
• delta toxin hemolytic

53
Examples of bacterial toxins and their mode of
action

• Toxin Source Mechanism of Action
• Botulinum toxin C. botulinum Act at presynaptic
  neuromuscular junctions and inhibits
  Ca2-dependent exocytosis of Ach containing
  vesicles, causes flaccid paralysis
• Tetanospasmin C. tetani Proteolytic activity,
  causes spastic paralysis due to inhibition of
  neurotransmitter release

54

• Botulism
• C. botulinum makes at least 7 neurotoxins (A-G).
  
• The toxins can be detected in feces, blood, and
  food source. Exposure is usually via ingestion
  of preformed toxins, but vegetative growth of
  bacteria in gut can occur.
• Ingestion of toxins
• Toxins move to blood
• Invasion of peripheral nerves (hours to days)
• Blocks release of acetylcholine resulting in
  flaccid paralysis, usually of ocular and
  respiratory muscles first
• Animals remain aware and can feel pain
• No primary lesions seen
• Limberneck in birds, quadraplegia in mammals
• blurred vision, swallowing difficulty
• mortality depends on species and on toxin dose
  recovery length depends on dose but can be as
  long as 1 month. Recovery is usually complete.

55

• Tetanus
• Cl. tetani spores enter deep tissues with low
  oxygen tension.
• Spores germinate and toxins are produced.
• In 3-21 days, tetanospasmin toxin blocks
  neurotransmitter (glycine and gamma-aminobutyric
  acid) release.
• Toxins spread along
• Peripheral nerves ascending tetanus (from wound
  to trunk)
• Hematogenous through lymph descending tetanus
  (lockjaw)
• Vascillating spasms occur respiratory distress
  victim remains conscious.
• Respiratory arrest, and death occur if toxin dose
  is high enough.

Toxins tetanospasmin- a neurotoxin that causes
clinical disease highly lethal hemolysin-
tissue necrosis nonspasmogenic toxin- function?
56

• Diptheria toxin C. diptheriae ADP-ribosylates
  elongation factor 2, stops protein synthesis
• Alpha-toxin S. aureus Forms transmembrane
  channels in a variety of host cells
• Cholera toxin V. cholera ADP ribosylates proteins
  involved in cAMP regulation
• Toxic shock S. aureus Stimulates cytokine
  syndrome toxin production by indiscriminately
  activating T cells

57
Superantigens
Superantigens overstimulate T cells by binding to
the MHC molecule and the TCR. Overproduction of
cytokines causes shock.
58
III. Opportunistic Pathogenesis
Cl. perfringens Type D produces alpha and
epsilon toxins associated with overeating
disease (pulpy kidney disease) in sheep, an
enterotoxemia The epsilon toxin is converted by
gastric juices, gets into the blood, and damages
kidneys. The brain can also be affected.
Diagnose by neural signs, no fever, no vomiting,
and history. Post-mortum, kidneys are swollen
and soft.
Sheep showing leaning and agonal behavior
associated with Cl. perfringens toxemia.
59

• Virulant foot rot
• Caused by the anaerobe, Bacteroides nodusus
• Pili are required for virulence.
• Data suggests a susceptibility link to the MHC.
• Enters wound in hoof from soil.
• Is aided by coinfection with Fusobacterium
  necrophorum and/or Actinomyces pyogenes, wet
  weather, or rocky terrain.
• Produces a protease which dissolves the keratin
  of the hoof.
• Enhances growth of F. necrophorum.
• The horn will rapidly separate from the foot
  very painful.
• Many animals recover spontaneously.

Normal (left) sheep hooves. The middle shows a
hoof during virulent foot rot infection. Note
the horn separation and matted tissue and and
soil. On right is the same hoof after cleaning
to raise the oxygen tension in the lesion.
60

• Dermatophilus congolensis
• Infection initiates with small nodules that crust
  over as exudate accumulates.
• Branched mycelia invade hair follicles and
  neutrophils accumulate under the dermis.
  Zoospores emerge when scabs become wet and spread
  to new sites on skin.
• Mortality is usually low, but morbidity is high.
• The organism spreads via contact or insects.
• Wet weather is associated with increased
  incidence, as are insect levels and poor
  nutrition.
• Host genetic susceptibility is noted, with
  European and African cattle most susceptible.

61
IV. Pathogenesis Induced by Host Immune Responses

• Lipopolysaccharide (LPS) is an integral part of
  the Gram-negative bacterial cell membrane. The
  lipid portion of LPS, lipid A, is responsible for
  endotoxic activity, and is located in the outer
  membrane of the bacterial cell. The
  oligosaccharide core along with the immunogenic
  "O" polysaccharide antigen project outward from
  the cell surface.
• Schematic representation of bacterial LPS and
  mechanism of action.

62
LPS Activation of Macrophages
63

• Actinomyces bovis- lumpy jaw
• Normally non-pathogenic, they are opportunistic
  invaders of deep tissues, usually introduced by a
  foreign body.
• Infection results in a dense mat of Actinomyces
  ( associated organisms) surrounded by
  neutrophils, macrophages, and plasma cells.
• Proteolytic enzymes from neutrophils and
  macrophages break down connective tissue and
  facilitate spread of the bacteria through
  tissues.
• Dense, fibrous masses are often observed, as are
  peritoneal or pleural exudates.

64
Rarefying osteomyletis with severe bone
destruction.
65

• Mycobacterium bovis
• Cause granulomatous lesions referred to as
  tubercles.
• The organism is inhaled or ingested. Neutrophils
  and macrophages engulf the organism.
• The organism blocks acidification of the
  phagolysosome.
• Proton ATPase is absent from vacuoles.
• The bacteria escape the phagolysosomes to
  replicate within host cells, initiating a slow
  inflammatory cascade.
• The surrounding tissue necrotizes.
• Epithelial cells surround the lesion, walling it
  off.
• Slowly, but eventually (months to years),
  connective tissue encompasses the lesion,
  impairing organ function.

necrotic center
fibroblasts and histiocytes
epithelial cells
Photomicrograph of a tubercule showing zone of
central necrosis surrounded by proliferating
fibroblasts and epithelial cells.
66

• Virulence Factors
• High lipid content- 60-70 lipids in outer
  membrane prevent drying/acids/bases.
• Cord Factor- trehalose 6-6 dimycolate Present
  in the cell wall membrane of all pathogenic
  strains.
• Sulfolipids- trehalose 2-sulfates esterified
  with long chain fatty acids. Lipids are believed
  to interfere with phagolysosome fusion.
• Mycobactins- siderophores.
• Protein P32- secreted protein that activates
  cytotoxic T cells to kill infected macrophages.
• Lipoarabinomannans (LAMs)- activate macrophage
  expression of TNF-? and reactive nitrogen
  products. Chemotactic for T cells.
• Taurine chloramine- inhibits cytokine expression
  by macrophages. It is a derivative of the amino
  acid taurine which is present in high levels in
  phagocytes.
• Induction of novel gene expression following
  phagocytosis.
• There are no exotoxins associated with virulence
  in Mycobacteria

67

• M. paratuberculosis
• Causative agent of Johnes disease, a chronic
  enteritis of cattle and other ruminants.
• Animals ingest materials contaminated with
  infected feces. Exposure is usually when animals
  are young during suckling.
• The organism penetrates the ileum and colon.
• Macrophages ingest the organism, but no
  phagolysosome fusion occurs.
• As disease progresses, clinical signs slowly
  appear
• Thickening of the intestinal wall due to
  epithelial cell proliferation
• Emaciation despite normal appetite
• Swelling of regional lymph nodes
• Coat becomes dry and rough
• Bottle jaw, a mandibular edema caused by
  protein loss
• Chronic or intermittent diarrhea

68
Sections of intestine from an animal suffering
Johnes disease.