Disease – Epidemiology and Control Introduction to Microbiology Chapters 14 and 20
Jan 02, 2016
Disease – Epidemiology and Control
Introduction to MicrobiologyChapters 14 and 20
Epidemiology
• The study of where and when diseases occur• Centers for Disease Control and Prevention
(CDC)– Collects and analyzes epidemiological information in
the United States– Publishes Morbidity and Mortality Weekly Report
(MMWR) – www.cdc.gov
John Snow 1848–1849 Mapped the occurrence of cholera in London
Ignaz Semmelweis 1846–1848 Showed that handwashing decreased the incidence of puerperal fever
Florence Nightingale
1858 Showed that improved sanitation decreased the incidence of epidemic typhus
Epidemiology
Epidemiology
• Descriptive: Collection and analysis of data– Snow
• Analytical: Comparison of a diseased group and a healthy group– Nightingale
• Experimental: Controlled experiments– Semmelweis
• Case reporting: Health care workers report specified disease to local, state, and national offices
• Nationally notifiable diseases: Physicians are required to report occurrence
Epidemiology
The CDC
• Morbidity: Incidence of a specific notifiable disease
• Mortality: Deaths from notifiable diseases• Morbidity rate: Number of people affected in
relation to the total population in a given time period
• Mortality rate: Number of deaths from a disease in relation to the population in a given time
Pathology, Infection, and Disease
• Pathology: The study of disease• Etiology: The study of the cause of a disease• Pathogenesis: The development of disease• Infection: Colonization of the body by pathogens• Disease: An abnormal state in which the body is
not functioning normally
Normal Microbiota and the Host
• Transient microbiota may be present for days, weeks, or months
• Normal microbiota permanently colonize the host
• Symbiosis is the relationship between normal microbiota and the host
Representative Normal Microbiota
Figure 14.1
Symbiosis
• In commensalism, one organism benefits, and the other is unaffected
• In mutualism, both organisms benefit• In parasitism, one organism benefits at the
expense of the other• Some normal microbiota are opportunistic
pathogens
Normal Microbiota and the Host
• Microbial antagonism is a competition between microbes.
• Normal microbiota protect the host by– Occupying niches that pathogens might occupy– Producing acids– Producing bacteriocins
• Probiotics: Live microbes applied to or ingested into the body, intended to exert a beneficial effect
Classifying Infectious Diseases
• Symptom: A change in body function that is felt by a patient as a result of disease
• Sign: A change in a body that can be measured or observed as a result of disease
• Syndrome: A specific group of signs and symptoms that accompany a disease
Classifying Infectious Diseases
• Communicable disease: A disease that is spread from one host to another
• Contagious disease: A disease that is easily spread from one host to another
• Noncommunicable disease: A disease that is not transmitted from one host to another
Occurrence of a Disease
• Incidence: Fraction of a population that contracts a disease during a specific time
• Prevalence: Fraction of a population having a specific disease at a given time
• Sporadic disease: Disease that occurs occasionally in a population
Occurrence of a Disease
• Endemic disease: Disease constantly present in a population
• Epidemic disease: Disease acquired by many hosts in a given area in a short time
• Pandemic disease: Worldwide epidemic• Herd immunity: Immunity in most of a
population
Severity or Duration of a Disease
• Acute disease: Symptoms develop rapidly• Chronic disease: Disease develops slowly• Subacute disease: Symptoms between acute and
chronic• Latent disease: Disease with a period of no
symptoms when the causative agent is inactive
Extent of Host Involvement
• Local infection: Pathogens are limited to a small area of the body
• Systemic infection: An infection throughout the body
• Focal infection: Systemic infection that began as a local infection
• Superinfection: occurs when a pathogen develops resistance to the drug being used for treatment or when normally resistant microbiota multiply excessively, adding to infection.
Extent of Host Involvement
• Sepsis: Toxic inflammatory condition arising from the spread of microbes, especially bacteria or their toxins, from a focus of infection
• Bacteremia: Bacteria in the blood• Septicemia: Growth of bacteria in the blood
Extent of Host Involvement
• Toxemia: Toxins in the blood• Viremia: Viruses in the blood• Primary infection: Acute infection that causes
the initial illness• Secondary infection: Opportunistic infection
after a primary (predisposing) infection• Subclinical disease: No noticeable signs or
symptoms (inapparent infection)
Predisposing Factors
• Make the body more susceptible to disease– Short urethra in females– Inherited traits, such as the sickle cell gene– Climate and weather– Fatigue– Age– Lifestyle– Chemotherapy
The Stages of a Disease
Figure 14.5
The Spread of Infection
• Reservoirs of Infection– Continual sources of infection
• Human: AIDS, gonorrhea– Carriers may have inapparent infections
or latent diseases
• Animal: Rabies, Lyme disease– Some zoonoses may be transmitted to humans
• Nonliving: Botulism, tetanus– Soil
The Spread of Infection – Transmission Types
• Contact• Vehicle• Vector
Transmission of Disease
• Contact– Direct: Requires close association between infected
and susceptible host– Indirect: Spread by fomites
• Fomite – inanimate object that can spread disease.– Example: toys, clothing, utensils, etc.
– Droplet: Transmission via airborne droplets
Transmission of Disease
• Vehicle– Contact with food, water, other liquids
• These are constantly taken into the body, so they serve as “vehicles” into the body.
Transmission of Disease
• Vector– Transmission from an animal (insect)
Transmission of Disease
Figure 14.6a, d
Vehicle Transmission
• Transmission by an inanimate reservoir (food, water, air)
Figure 14.7b
Nosocomial Infections
• Are acquired as a result of a hospital stay• Affect 5–15% of all hospital patients
Figure 14.6b
Nosocomial Infections
Figure 14.9
Nosocomial Infections
Table 14.5
Percentage of Total Infections
Percentage Resistant to Antibiotics
Coagulase-negative staphylococci
25% 89%
S. aureus 16% 80%
Enterococcus 10% 29%
Gram-negative rods 23% 5-32%
C. difficile 13% None
Common Causes of Nosocomial Infections
MRSA
• USA100: 92% of health care strains • USA300: 89% of community-acquired strains
Clinical Focus, p. 422
Which Procedure Increases the Likelihood of Infection Most?
Clinical Focus, p. 422
Emerging Infectious Diseases
• Diseases that are new, increasing in incidence, or showing a potential to increase in the near future
Emerging Infectious Diseases
• Contributing factors– Genetic recombination
• E. coli O157, avian influenza (H5N1)
– Evolution of new strains• V. cholerae O139
– Inappropriate use of antibiotics and pesticides• Antibiotic-resistant strains
– Changes in weather patterns• Hantavirus
Emerging Infectious Diseases
• Modern transportation– West Nile virus
• Ecological disaster, war, and expanding human settlement– Coccidioidomycosis
• Animal control measures– Lyme disease
• Public health failure– Diphtheria
Crossing the Species Barrier
Clinical Focus, p. 371
Treatment – Antibiotics/Antimicrobials
• Chemotherapy: The use of drugs to treat a disease
• Antimicrobial drugs: Interfere with the growth of microbes within a host
• Antibiotic: A substance produced by a microbe that, in small amounts, inhibits another microbe
• Selective toxicity: A drug that kills harmful microbes without damaging the host
• 1928: Fleming discovered penicillin, produced by Penicillium
• 1940: Howard Florey and Ernst Chain performed first clinical trials of penicillin
Antimicrobial Drugs
Figure 1.5
Most Antibiotics come from other Microorganisms
The Spectrum of Antimicrobial Activity
The Spectrum of Antimicrobial Activity
• Broad spectrum – Treats a variety of microbes
• Narrow spectrum– Treats few microbes
• Bactericidal– Kill microbes directly
• Bacteriostatic
– Prevent microbes from growing
• Ideally, an antimicrobial should target the pathogen only and not disrupt normal microbiota
– Risk vs. Benefit Analysis
The Action of Antimicrobial Drugs
The Action of Antimicrobial Drugs
Commonly used Antimicrobials – Modes of Action
- Inhibitors of Cell Wall Synthesis- Antimycobacterial Antibiotics- Inhibitors of Protein Synthesis - Injury to the Plasma Membrane - Inhibitors of Nucleic Acid (DNA/RNA)
Synthesis- Competitive Inhibitors of the Synthesis of
Essential Metabolites
Commonly Used Antimicrobials
• Antibacterial Antibiotics: Inhibitors of Cell Wall Synthesis– All penicillins (natural and semisynthetic)– Carbapenems– Cephalosporins– Bacitracin – Vancomycin
Commonly Used Antimicrobials
• Antimycobacterial Antibiotics– Isoniazid (INH) and ethambutol inhibit cell wall
synthesis in mycobacteria.
Commonly Used Antimicrobials
• Inhibitors of Protein Synthesis – Chloramphenicol, aminoglycosides, tetracyclines,
macrolides, and streptogramins inhibit protein synthesis at 70S ribosomes.
– Oxazolidinones prevent formation of 70S ribosomes.
Commonly Used Antimicrobials
• Injury to the Plasma Membrane – A new class of antibiotics inhibits fatty-acid
synthesis, essential for plasma membranes.• Polymyxin B and bacitracin cause damage to plasma
membranes.
Commonly Used Antimicrobials
• Inhibitors of Nucleic Acid (DNA/RNA) Synthesis
• Rifamycin inhibits mRNA synthesis; it is used to treat tuberculosis.
• Quinolones and fluoroquinolones inhibit DNA gyrase for treating urinary tract infections.
Commonly Used Antimicrobials
• Competitive Inhibitors of the Synthesis of Essential Metabolites– Sulfonamides competitively inhibit folic acid
synthesis.
Antiviral Drugs
• Protease inhibitors– Indinavir: HIV
• Integrase inhibitors– HIV
• Inhibit attachment– Zanamivir: Influenza– Block CCR5: HIV
• Inhibit uncoating– Amantadine: Influenza
• Nucleoside and Nucleotide Analogs– Inhibit DNA/RNA synthesis
• Enzyme inhibitors– Inhibit fusion, Inhibit attachment, Inhibit uncoating
• Interferons– Prevent spread of viruses to new cells
• Metronidazole
– Damages DNA– Entamoeba, Trichomonas
• Nitazoxanide
– Interferes with metabolism of anaerobes
Antiprotozoan Drugs
Antihelminthic Drugs
• Niclosamide– Prevents ATP generation
• Tapeworms
• Praziquantel– Alters membrane
permeability• Flatworms
Figure 12.26
Antihelminthic Drugs
Figure 12.28a
• Mebendazole– Inhibits nutrient
absorption• Intestinal roundworms
• Ivermectin– Paralyzes worm
• Intestinal roundworms
Testing Effectiveness of Treatment
• Disk Diffusion Test (Kirby Bauer)• Broth Dilution Test• MIC (minimal inhibitory concentration) Test
– The E-test– Microtitre plates
The Disk-Diffusion Method
Figure 20.17
The E Test
Figure 20.18
Microtitre Plate
Figure 20.19
Antibiotic Resistance
• A variety of mutations can lead to antibiotic resistance
• Mechanisms of antibiotic resistance1. Enzymatic destruction of drug2. Prevention of penetration of drug3. Alteration of drug's target site4. Rapid ejection of the drug
• Resistance genes are often on plasmids or transposons that can be transferred between bacteria
Antibiotic Resistance
• Misuse of antibiotics selects for resistance mutants. Misuse includes– Using outdated or weakened antibiotics– Using antibiotics for the common cold and other
inappropriate conditions– Using antibiotics in animal feed– Failing complete the prescribed regimen– Using someone else's leftover prescription
Effects of Combinations of Drugs
• Synergism occurs when the effect of two drugs together is greater than the effect of either alone
• Antagonism occurs when the effect of two drugs together is less than the effect of either alone
Synergism between Two Different Antibiotics
Figure 20.23
Antagonism Between Antimicrobials – The D-test
The Future of Antimicrobial Treatment and Development
• Chemicals produced by plants and animals are providing new antimicrobial agents called antimicrobial peptides.
• Phage therapy is also something that is being investigated for new treatment