Modern Infectious Diseases: Scope and Impact

• Microbiology's scope includes basic and systems-based study of infectious diseases.
• Curriculum will cover microorganisms, host interactions, common pathogens, antimicrobial agents, investigating infections, and infection control.
• Expectations involve understanding organism names, antimicrobial agents, and related principles.
• It is important to build a knowledge base of common pathogens and related diseases.

Modern Infectious Diseases

• In 2019, 85 pathogens caused 28% of global DALYs (Disability-Adjusted Life Years).
• Sub-Saharan Africa (SSA) accounted for 62% of these DALYs, while high-income regions accounted for 10%.
• The top three diseases contributing to DALYs are TB, Malaria, and HIV.
• Malaria is the leading cause of disease burden in children under 5.
• Gram-positive and Gram-negative organisms contribute significantly to the infectious disease burden.

Pathogen Classification and Genomic Sizes

• Viruses are classified as DNA or RNA (single-stranded or double-stranded), possessing envelopes or not.
• Examples of Viruses include Influenza (13.5kb) and HIV (9-10kb).
• Procaryotes include bacteria, mycobacteria, and actinomycetes.
• Examples of Procaryotes include E.coli (4.5-5.5Mb) and M.tuberculosis (~4Mb).
• Eucaryotes include fungi, protozoa, and helminths.
• Examples of Eucaryotes include C.albicans (~14Mb), Plasmodium falciparum (~23Mb), and O. volvulus (97Mb).
• For comparison, the human genome (Homo Sapiens) is 3 billion base pairs (3Bn).

Incubation Periods

• Incubation periods vary from hours to months.
• Short incubation periods include Cholera (2 hours to 5 days) and Influenza (1-3 days).
• Intermediate incubation periods include Chickenpox (11-20 days) and Malaria (10-15 days).
• Long incubation periods include Hepatitis B (6 weeks-6 months) and TB (1-12 months).

Modes of Transmission

• Direct transmission occurs via direct contact, droplets, exposure to infectious agents in the environment, bites, or transplacental/perinatal routes.
• Indirect transmission involves biological vectors or mechanical vectors, vehicles, and airborne transmission.

Exogenous Encounters

• Zoonotic transmission involves animal vectors like Rabies, toxoplasmosis, and brucellosis.
• Insect vectors include mosquitoes (Malaria, dengue) and ticks (tick-bite fever).
• Human transmission occurs through sexual contact (HIV, chlamydia), airborne particles (Varicella/chickenpox, TB), droplets (S.pneumoniae, N.meningitidis), fecal-oral route (shigellosis, typhoid fever), vertical transmission (CMV, listeriosis, syphilis), and fomites/direct hand contact (multidrug-resistant organisms in hospitals).

Endogenous Encounters

• E.coli is a part of the normal flora.
• Some E.coli strains have colonization factors (e.g., P-pili), while others have virulence factors causing diarrheal disease.
• Candida in HIV patients can lead to thrush and oesophagitis.
• Chronic CMV infection alongside immune suppression results in colitis and pneumonia.
• Antibiotics can disrupt normal flora, leading to pseudomembranous colitis and mucosal candidiasis.

Bacterial Structure - Key Components

• The cell wall is composed of peptidoglycan, a sugar backbone cross-linked by transpeptidase, providing rigidity.
• Outer membrane: Features in Gram-negative bacteria, containing LPS and porins for transport.
• Flagellum: Responsible for motility.
• Pili/Fimbriae: Facilitates adherence of bacteria to cell surfaces and forms sex pilus during conjugation.
• Capsule: An outer polysaccharide layer that resists phagocytosis.
• Spore: A keratin-like coat that resists dehydration, heat, and chemicals.

Gram Positive vs Gram Negative Bacteria - Key Differences

• Gram-positive bacteria contain lipoteichoic and teichoic acids within their cell walls
• Gram-negative bacteria have lipopolysaccharides (LPS)
• Gram-positive bacteria have a thicker peptidoglycan layer compared to Gram-negative bacteria.
• Gram-positive bacteria lack an outer membrane, unlike Gram-negative bacteria.
• Gram-negative bacteria possess a large periplasmic space between their inner and outer membranes, which Gram-positive bacteria lack.

Important Terms and Characteristics of Bacteria

• Most bacteria have cell walls, except for Mycoplasma spp.
• Bacteria are often extracellular but some are intracellular.
• Examples of obligate intracellular bacteria include Chlamydiae.
• Examples of Facultative intracellular bacteria include M.tuberculosis, N.gonorrhoeae.
• Bacteria can be: Facultative (most); Anaerobic (Bacteroides spp); Microaerophilic (Campylobacter spp, Helicobacter pylori).
• Encapsulation in bacteria like S.pneumoniae and H.influenzae helps avoid phagocytosis.

Genetic Diversity and Morphology

• Bacteria acquire new traits via horizontal gene transfer, a key factor in antibiotic resistance.
• Vertical gene transfer passes traits to offspring, while plasmids facilitate auto-replication of DNA.
• Cylindrical (bacilli), spherical (cocci), and spiral (spirochetes) forms are distinct bacterial morphologies.
• Gram staining identifies bacteria: positive strains retain crystal violet, staining blue, while negative strains stain red. Specialized stains, like Ziehl-Neelsen or fluorescence stains, are used for specific bacteria.

Overview of modern infectious diseases, their global impact, and classification. Focus on major pathogens, disease burden in different regions, and key contributors like TB, Malaria, and HIV. Gram-positive and Gram-negative organisms are discussed.