Characteristics of Bacteria causing Disease in Host

Describe Koch’s Postulate (principal)

• Identify suspected agent in all cases of the disease

• Isolate and grow the agent in pure culture on artificial media

• Introduce the pure culture into a healthy host to see if same disease appears

• If it does, re-isolate the same agent from the experimentally infected host to make sure

Bacteria 

• Compare size and self sufficiency to viruses 

• Types of shape 

• How many sets of chromosomes 

• How do they reproduce

• How mutations affect bacteria 

• Can bacteria acquire genes from other bacteria 

• How does bacterial evolution allow them to do

Compare size and self sufficiency to viruses:

• Size = 10 - 100 larger 

• Self sufficiency = More self sufficient

Types of shape: 

• Cocci (spherical)

• Rods (bacilli) 

• Spirochetes (spiral) 

How many sets of chromosomes: One set (haploid)

How do they reproduce: Binary fission producing clones

How mutations affect bacteria: Enhance survival

Can bacteria acquire genes from other bacteria: Yes, through horizontal gene transfer

How does bacterial evolution allow them to do: Adapt to new or existing environments to increasing survivability 

What are the 2 types of bacteria?

• Intracellular bacteria

• Extracellular bacteria

Intracellular Bacteria

• How do they enter host cells?

• How they survive inside host cells?

• Examples?

How do they enter host cells: By various pathways such as =

• Macrophagess

• Phagocytes

• Epithelial/endothelilal cells

• Hepatocytes

How they survive inside host cells: They evade the host’s immune response and replicate within mononuclear phagocytes

Examples:

• Brucella abortus

• Listeria monocytogenes

• Chalmydia trachomatis

• Coxiella brunteii

• Mycobacterium tuberculosis

• Salmonella enterica

Extracellular Bacteria

• Where do they multiply

• How do they evade immune system

• Can they enter host cells 

• Examples

• Examples of diseased caused by extracellular bacteria 

Where do they multiply: In extracellular spaces like mucosal surfaces, vascular and lymphatic fluids, body cavities

How do they evade immune system: They evade humoral immunity and phagocytes causing multiplication outside host cells

Can they enter host cells: Sometimes but most stay attached to epithelial surfaces and release toxins without penetration

Examples:

• Staphylococcus aureus 

• Streptococcus pyogenes 

• Pseudomonas aeruginosa 

• Escherichia coli 

Examples of diseased caused by extracellular bacteria: 

• Wound infections

• Osteomyelitis 

• Pneumonia  

• UTI 

Pathogenicity of Bacteria

• What 

• What does virulence mean

• Difference between avirulent and virulent bacteria 

• What are bacterial virulence factors

• What does bacterial pathogenicity depend on

What: The ability to cause disease and symptoms depending on virulence

What does virulence mean: How dangerous the bacteria are

Difference between avirulent and virulent bacteria:

• Avirulent: Little impact on health

• Virulent: Actively express virulence genes and cause health problems

What are bacterial virulence factors:

• Toxins

• Surface coats

• Surface receptors

What does bacterial pathogenicity depend on: Pathogen’s resistance to host defenses and host susceptibility to bacterial virulence factors

What are exotoxins and endotoxins?

They are virulence factors that help bacteria cause disease

Exotoxins

• How they cause disease

• Are they diffusible or cell bound 

• What type of molecules are exotoxins 

• Which bacteria produces exotoxins 

• Are they species specific 

• How dangerous are they

• Are they stable

• Can they be converted to a safe form for vaccines

How they cause disease: Directly damage host cells or interfere with normal cellular functions

• Acts extracellulary or on cell membrane

• Attack intercellular substances by enzymatic mechanisms (hemolysins, leukocidins, hyaluronidase)

• Proteins enter cells and disrupt cellular processes enzymatically

Are they diffusible or cell bound: Diffusible (can move away from bacterial cell that produces them and spread to surrounding tissues)

What type of molecules are exotoxins: Proteins or polypeptides

Which bacteria produces exotoxins: Both Gram-positive and Gram-negative bacteria

Are they species specific: Yes

How dangerous are they: Lethal in small amounts (nanograms) 

Are they stable: No they are liable to heat, chemicals and storage conditions

Can they be converted to a safe form for vaccines: Yes, they can be converted to toxoids that initials antitoxin production

Endotoxins

• How they cause disease 

• Are they diffusible or cell bound 

• What is the toxic component 

• Which bacteria produce endotoxins 

• Are they species specific 

• How dangerous are they 

• Are they stable 

• Can they be converted to toxoids

How they cause disease: Trigger strong immune responses 

• Enter host cells in macrophages

• Stimulates the secretion of mediator substances such as interleukin-1, tumor necrosis factor (TNF) and complement components

Are they diffusible or cell bound: Cell bound (part of bacterial cell wall)

What is the toxic component: Lipid A of lipopolysaccharide (LPS) 

Which bacteria produce endotoxins: Only Gram negative 

Are they species specific: No, they are structurally similar and have similar effects regardless of bacterial species

How dangerous are they: Lethal in large amounts (micrograms) 

Are they stable: Yes, very stable to heat, chemicals and storage

Can they be converted to toxoids: No, they cannot be converted to toxoids

What is pathogenic action?

The specific way the microorganism causes disease in the host

(different from pathogenicity which means the ability of a microorganism to cause disease)

What are the 2 types of pathogenic action of bacteria?

• Direct damage to host structures and functions by exotoxins

• Damage occurred as a result of host immune responses following exposure to endotoxins

What is immune mediate damaged by endotoxins?

Tissue damage occurs due to immune reactions

• Acute: Inflammation resembling immediate type allergy 

• Chronic: Granulomatous infections cause cell mediated responses that intensify inflammation and tissue destruction 

Antibiotics: 

What bacterial mechanisms do antimicrobial agents target?

• Inhibition of cell wall synthesis

• Disruption of protein synthesis

• Impairment of nucleic acid synthesis

• Interference with metabolic pathways

Antibiotics: 

How can bacteria resist antimicrobial agents?

• Prevent drug entry

• Deactivate drugs

• Alter drug targets

• Actively expel drugs

Antibiotics:

How does mutation and natural selection lead to antibiotic resistance?

• Susceptible bacteria die

• Mutations create resistant bacteria that survive.

• Resistant bacteria reproduce, dominating the population.

Antibiotics:

What is horizontal gene transfer in antibiotic resistance?

Bacteria acquire resistance genes from other bacteria instead of through mutation

Antibiotics:

What is conjugation?

Transfer of DNA (usually plasmids) from one bacterium to another via direct contact

Antibiotics:

What is transduction?

A bacteriophage transfers resistance genes from one bacterium to another

Antibiotics:

What is transformation?

Bacteria pick up free-floating DNA from the environment, often from dead bacteria, acquiring resistance genes

Pathogenesis

What is the first step in bacterial pathogenesis?

Exposure – the host comes into contact with bacteria via skin, mucosa, inhalation, ingestion, or other routes

Pathogenesis

What happens after exposure in bacterial pathogenesis?

Colonization – bacteria attach to host surfaces (epithelial cells, mucosa) and start multiplying.

Pathogenesis

How do bacteria evade the host immune system?

Immune evasion – bacteria avoid detection or destruction via capsules, toxins, biofilms, or intracellular survival

Pathogenesis

What is the result of successful immune evasion?

Infection – bacteria multiply, spread, and cause tissue damage, leading to disease.

Pathogenesis

What is the symptomatic phase of bacterial disease?

• Bacterial exposure

• Colonisation

• Immune evasion

Pathogenesis

What is the asymptomatic phase of bacterial disease?

Infection

What are the factors affecting bacterial pathogenesis?

• Host immune system

• Host genetics 

• Age

• Host microbiota

• Coexisting infections

• Pathogen virulence 

• Pathogen resistance