Antimicrobial Drugs and Infectious Diseases: Mechanisms, Resistance, and Epidemiology

Antimicrobial Drugs: Mechanisms and Resistance

Historical Contributions to Antimicrobial Development

• Paul Ehrlich: Developed the concept of the "magic bullet" and discovered Salvarsan, the first chemotherapeutic agent effective against syphilis.

• Alexander Fleming: Discovered penicillin, the first true antibiotic, from the mold Penicillium notatum.

• Gerhard Domagk: Discovered prontosil, the first sulfa drug, which was effective against bacterial infections.

Principle of Selective Toxicity

• Selective toxicity refers to the ability of an antimicrobial drug to harm the pathogen without causing significant damage to the host.

• This principle is based on differences in cellular structure or metabolism between microbes and host cells.

Mechanisms of Antimicrobial Action

Antimicrobial drugs target pathogens through six main mechanisms:

• Inhibition of cell wall synthesis: Prevents bacteria from forming peptidoglycan, leading to cell lysis (e.g., penicillins, cephalosporins).

• Inhibition of protein synthesis: Targets bacterial ribosomes, disrupting protein production (e.g., tetracyclines, aminoglycosides).

• Disruption of cytoplasmic membrane: Damages membrane integrity, causing cell contents to leak (e.g., polymyxins, amphotericin B).

• Inhibition of metabolic pathways: Blocks key enzymatic reactions (e.g., sulfonamides inhibit folic acid synthesis).

• Inhibition of nucleic acid synthesis: Interferes with DNA or RNA synthesis (e.g., quinolones, rifampin).

• Inhibition of pathogen attachment or entry: Blocks adhesion or entry into host cells (e.g., some antiviral drugs).

Narrow-Spectrum vs. Broad-Spectrum Drugs

• Narrow-spectrum drugs: Effective against a limited range of microbes; reduces risk of disrupting normal microbiota but may miss the pathogen.

• Broad-spectrum drugs: Effective against a wide variety of microbes; useful for unknown infections but may cause secondary infections or superinfections by disrupting normal microbiota.

Routes of Administration of Antimicrobial Drugs

• Oral: Convenient but may result in lower and slower absorption.

• Intramuscular (IM): Allows higher concentrations but requires injection.

• Intravenous (IV): Provides highest and most rapid drug levels but requires medical supervision.

Advantages and disadvantages depend on the drug, infection site, and patient condition.

Microbial Antagonism, Normal Microbiota, and Secondary Infections

• Microbial antagonism: The competition between normal microbiota and pathogens for nutrients and space, helping prevent infections.

• Normal microbiota: The collection of microorganisms regularly found at various sites in the body.

• Secondary infections: Infections that occur when normal microbiota are disrupted, often by broad-spectrum antibiotics.

Development of Antimicrobial Resistance

• Populations of resistant microbes arise through natural selection when exposed to antimicrobial agents.

• Genetic changes (mutations or acquisition of resistance genes) allow survival and proliferation in the presence of drugs.

R Plasmids and Resistance

• R plasmids (resistance plasmids): Extrachromosomal DNA elements that carry genes for antimicrobial resistance and can be transferred between bacteria.

• Cells with R plasmids can survive drug exposure and spread resistance.

Mechanisms of Microbial Resistance

Microorganisms can resist antimicrobial drugs by seven main mechanisms:

• Enzymatic destruction or inactivation of the drug (e.g., beta-lactamases).

• Alteration of drug target site.

• Decreased uptake of the drug into the cell.

• Increased efflux (pumping out) of the drug.

• Bypass of the metabolic pathway inhibited by the drug.

• Formation of biofilms that impede drug penetration.

• Target overproduction or protection.

Cross Resistance vs. Multiple Resistance

• Cross resistance: Resistance to multiple drugs with similar structures or mechanisms (e.g., all beta-lactam antibiotics).

• Multiple resistance: Resistance to several unrelated drugs, often due to accumulation of resistance genes.

Retarding Development of Resistance

Four main strategies to slow resistance:

• Use high concentrations of drugs for appropriate durations.

• Use combination therapy (multiple drugs).

• Limit use of antimicrobials to necessary cases.

• Develop new drugs and modify existing ones.

Infection, Infectious Diseases, and Epidemiology

Symbiosis and Types of Symbiotic Relationships

• Symbiosis: A close association between two different species.

• Types:

  • Mutualism: Both organisms benefit.

  • Commensalism: One benefits, the other is unaffected.

  • Parasitism: One benefits at the expense of the other.

Microbiome: Resident vs. Transient Microbiota

• Microbiome: The total collection of microorganisms in a particular environment, such as the human body.

• Resident microbiota: Microbes that are permanently present.

• Transient microbiota: Microbes present temporarily.

Acquisition of the Human Microbiome

• Humans acquire their microbiome during birth and through contact with the environment, food, and other people.

Opportunistic Pathogens

Three conditions that create opportunities for normal microbiota to cause disease:

• Immune suppression.

• Changes in normal microbiota (e.g., antibiotic use).

• Introduction of microbiota into unusual body sites.

Reservoirs of Infection

• Animal reservoirs: Other animals harboring pathogens.

• Human carriers: Infected individuals who may not show symptoms.

• Nonliving reservoirs: Soil, water, and food.

Contamination, Infection, and Disease

• Contamination: Presence of microbes on surfaces or tissues.

• Infection: Microbes invade and multiply in the host.

• Disease: Infection results in damage or dysfunction of host tissues.

Portals of Entry for Pathogens

• Skin (cuts, abrasions, bites).

• Mucous membranes (respiratory, gastrointestinal, urogenital tracts).

• Placenta (rare but possible).

• Parenteral route (direct deposition into tissues beneath the skin or mucous membranes).

Adhesion Factors and Their Role in Infection

• Adhesion factors: Structures or molecules (e.g., fimbriae, adhesins) that allow pathogens to attach to host cells, a critical step in establishing infection.

Pathogenicity and Virulence

• Pathogenicity: The ability of a microbe to cause disease.

• Virulence: The degree of pathogenicity; often measured by the severity of disease or the infectious dose required.

Symptoms, Signs, and Syndromes

• Symptoms: Subjective effects felt by the patient (e.g., pain, fatigue).

• Signs: Objective, measurable changes (e.g., fever, rash).

• Syndrome: A group of symptoms and signs that characterize a disease.

Koch’s Postulates and Their Limitations

• Koch’s postulates are criteria for establishing a causative relationship between a microbe and a disease.

• Limitations include inability to culture some microbes, ethical concerns, and diseases caused by multiple pathogens or by normal microbiota under certain conditions.

Virulence Factors

• Extracellular enzymes: Aid in invasion and evasion of host defenses.

• Toxins: Damage host tissues (e.g., exotoxins, endotoxins).

• Adhesion factors: Facilitate attachment to host cells.

• Antiphagocytic factors: Help evade immune system (e.g., capsules, M protein).

Stages of Infectious Disease

1. Incubation period: Time between infection and appearance of symptoms.

2. Prodromal period: Early, mild symptoms.

3. Illness: Most severe signs and symptoms.

4. Decline: Symptoms subside as immune response or treatment takes effect.

5. Convalescence: Recovery and return to normal function.

Modes of Transmission

• Contact transmission: Direct, indirect, or droplet spread.

• Vehicle transmission: Through air, water, or food.

• Vector transmission: Via insects or other animals (mechanical or biological vectors).

Classification of Diseases (Table 14.12)

Classification	Description
Acute	Develops rapidly, lasts a short time (e.g., influenza)
Chronic	Develops slowly, persists over a long period (e.g., tuberculosis)
Subacute	Intermediate between acute and chronic
Latent	Pathogen remains inactive for a time, then becomes active (e.g., herpes)
Communicable	Can be spread from host to host
Noncommunicable	Not spread from host to host

Epidemiology: Incidence and Prevalence

• Epidemiology: The study of the occurrence, distribution, and control of diseases in populations.

• Incidence: Number of new cases in a given time period.

• Prevalence: Total number of cases (new and existing) at a given time.

Patterns of Disease Occurrence

• Endemic: Disease constantly present in a population.

• Sporadic: Occurs occasionally and irregularly.

• Epidemic: Sudden increase in cases above normal expectations.

• Pandemic: Epidemic that spreads across continents or worldwide.

Additional info: Academic context and definitions have been expanded for clarity and completeness.