Lecture #1-2

Introduction to Microbial Pathogens

Fig 20.1. Community water supply in a developing country

Recent microbes in the news

• Severe acute respiratory syndrome (SARS)

• West Nile virus• Viral or bacterial meningitis• Human immunodeficiency virus (HIV)

• Tuberculosis (drug resistant)

• Cholera and Malaria • Bovine sphongiform encephalitis (Madcow

disease)

• Salmonella

Microbial infection and pathogenesis

Types of microbes

- Bacteria, fungi and protozoa

- Viruses, viroids, prions

Pathogenesis - interactions of molecular events in replication of a microbe and host responses that can result in disease

Figs 1.5, 1.7, 1.9

Figs 1.10 and 1.11

Microbial infection + immune response = pathogenesis

• Patterns of infection or disease

. acute - short-lived

. persistent - continuous

. latent - reoccurs

. subclinical - carrier state, no symptoms

Common terms in epidemiologyTable 20.1

Unifying concepts for replication and pathogenesis

Microbe structure (eg. morphology, genome type and size, species or variant) • determines how it interacts with- . host or host cells to replicate • . the host immune response

• Knowledge of the interaction of microbe with cells provides insights into:• clinical manifestations of infections and• how to control or interfere with these

Impact of viruses

• Many discovered in the last 20 years

• Viral infections cause estimated 50% of all absenteeism from work and school

• Bacteriophages affect drug resistance and molecular biology eg. restriction enzymes and reverse transcriptase for cloning

• Study as tools to explore biological processes

Three things all microbes must do

– 1 - Make more progeny– 2 - Spread and transmission– 3 - Evade host defenses

• Outcomes of these determine

• pathogenesis

Fig 20.1

Spread of pathogens

How do these microbes spread?

• Severe acute respiratory syndrome (SARS)

• West Nile virus• Viral or bacterial meningitis• Human immunodeficiency virus (HIV)

• Tuberculosis (drug resistant)

• Cholera and Malaria • Bovine sphongiform encephalitis (Madcow

disease)

• Salmonella

Reservoirs of infectious agents

• Carriers (asymptomatic or subclinical)

• Zoonotic diseases

• Environmental

Fig. 20.01a

Reservoirs harborpotentially pathogenic microbes

Transmission

• Modes of transmission to humans

- human to human

- animal to human

- insect to human

• Infection can be localized or systemic

- replicates, remains in local area of entry - replicates, spreads by viremia to other sites

Fig. 20.01b

Modes of transmission of microbes

Modes of transmission to or from a host

Sites of microbe entry or shedding

– Respiratory (secretions, aerosols)

– Oral/enteric (food, water)

– Urogenital (sexually transmitted)

– Vectors (insects, needles, animals)

– Contaminated tissues or body products

Fig. 20.01c

Portals of entry

Some specific modes of virus transmission

Skin infections - for most, skin lesions not significant means of

transmission

- exceptions are HSV in genital herpes, chicken pox from shingles, small pox in dried crusts- infectious for months, up to a year

Respiratory tract infections- transmission in air depends on coughing, sneezing

or infected secretions

Air bourne transmissionFigure 20.3

Some specific modes of virus transmission (cont.)

Semen• - HIV- much less for CMV, hepatitis B

- Human milk or colostrum• - CMV, HTLV by mother to child • - not a major transmission mode for• hepatitis B, encephalitis viruses, mumps,

rubella

Some specific modes of virus transmission (cont.)

Salivary secretions• - EBV, rabies- rare possibility for CMV, hepB

- Gastroenteric transmission- stools eg. enteric virus, poliovirus, - rotavirus, hepatitis A• - childcare centers, institutions, military camps• - contaminated water from poor waste disposal• - urine is not a major means of transmission

Host factors that affect susceptibility

• age of host• underlying physiological conditions• malnutrition• genetic determinants• gender• environmental conditions

• others eg. stress, personal behavior

Trends in disease• Reduction and eradication of disease

• Emerging diseases

Epidemiology• The study of factors that influence

disease frequency and distribution

Fig. 20.08

National and worldwide surveillance of infectious diseases is critical

Fig. 20.09Fig 20.9

Child with

smallpox,

an

eradicated

disease (polio,

bubonic

plague)

Fig 20.10 World Map of Emerging Diseases

Nosocomial infections

Figure 20.11

Fig. 20.12

Nosocomial infections

• Hospital acquired disease

• Hospitals are reservoirs of infectious agents

• Hospitals enable transmission of infectious agents

• How to prevent nosocomial infections ?

Reservoirs

• Other patients

• Hospital environment

• Health care workers

• Patient’s own normal flora

What procedures are used in dentistry?

Survival strategies of microbes

• Gain entry into host• Multiply at local site• Find suitable niche• Overcome or subvert host defenses

- outrun

- antigenic change

- hide in host

- mimic host component

- inactivate/down-regulate host response

? Questions to consider

• What is the clinical disease manifestation?• What microbe(s) causes the disease(s)?• How does the microbe enter or leave the host?• What is the target tissue(s) and means of

replication?• Is there damage from replication or immune

response?• What are the disease patterns?• What are the controls, preventions or therapies?• Specific distinguishing features