Prokaryotes vs Eukaryotes

Prokaryotes

• They lack a distinct nucleus
• Their DNA is in a single, circular chromosome
• Additional DNA is in plasmids
• Transcription and translation occur simultaneously

Eukaryotes

• DNA is carried on several chromosomes within a nucleus
• A nuclear membrane bounds the nucleus
• Transcription requires producing messenger RNA (mRNA) and moving it out of the nucleus into the cytoplasm
• Translation occurs on ribosomes
• Cytoplasm has membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes

Bacterial Cell Structures and Functions

• Capsule: An external coating that protects the cell, aids adherence, protects against phagocytosis, and has antigenic function
• Cell Wall: Determines cell shape, confers rigidity, and forms a physical barrier against the outside environment; its rigid component is peptidoglycan
• Flagellum: A specialized, unbranched, filamentous, helical appendage attached to the cell by a basal body, enabling rotation for motility
• Cytoplasm Gel containing enzymes, ions, and granules acting as food and energy reserves
• Mesosomes: Help during cell division and are centers for electron transport and photophosphorylation
• Bacterial chromosome/nucleoid: Made of condensed DNA that directs genetics and heredity
• Ribosomes: Sites of protein synthesis
• Pili/fimbriae: Hollow, non-filamentous appendages shorter than flagella, important for attachment (adhesin), bacterial communication, biofilm formation, genetic material exchange, and bacteriophage attachment

Gram Stain

• Gram-positive bacteria stain purple or blue
• Gram-negative bacteria stain pink or red

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-positive bacteria have a thick peptidoglycan layer with teichoic and lipoteichoic acids
• Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide, phospholipids, and proteins

Gene Transfer and Recombination

• New genotypes occur when genetic material transfers from bacteria
• Transferred DNA recombines or resides on a replicating plasmid
• Recombination can cause significant genetic changes and express functional genes phenotypically
• DNA can transfer from donor to recipient cells through transformation, transduction, conjugation, or transposition

Transformation

• Bacteria can transform by taking in DNA from their surroundings
• Certain bacteria are naturally 'competent' to take up DNA fragments from related species: Streptococcus pneumoniae and Haemophilus influenzae

Conjugation

• Is a bacterial 'mating'; where DNA transfers from one bacterium to another via a sex pilus

Transduction

• It involves transferring genetic material through bacteriophage infection

Gram-Positive Bacteria Examples

• Staphylococcus aureus
• Staphylococcus epidermidis
• Staphylococcus saprophyticus
• Streptococcus pyogenes
• Streptococcus pneumoniae
• Viridans Streptococcus
• Corynebacterium diphtheriae

Gram-Negative Bacteria Examples

• Vibrio cholerae
• Helicobacter pylori
• Pseudomonas auroginosa
• Acinetobacter baumanni
• E. coli
• Salmonella spp
• Neisseria meningitidis
• Neisseria gonorrhoeae

Staphylococcus

• Gram-positive cocci in clusters
• Divided into coagulase-negative (S. epidermidis, S. saprophyticus) and coagulase-positive (S. aureus) groups

S. aureus Epidemiology

• Normal flora on human skin and mucosal surfaces that can survive on dry surfaces for long periods
• Spreads through direct contact or contaminated fomites
• Risk factors include foreign bodies or antibiotics that supress the normal microbial flora

Streptococci Classification Based on Hemolysis on Blood Agar

• α-hemolysis: Partial hemolysis, green discoloration around colonies

  • Example: non-groupable streptococci (S. pneumoniae)

• β-hemolysis: Complete hemolysis, clear zone around colonies

  • Example: Group A and B

• γ-hemolysis: No lysis

  • Example: Group D (Enterococcus spp)

Group A Streptococci (S. pyogenes)

• Causes streptococcal infections, with peak incidence at 5-15 years of age
• 90% of cases cause pharyngitis

Enterobacteriaceae Virulence Factors

• O antigen: The outer membrane lipopolysaccharide (LPS)
• K antigen: The cell surface polysaccharides forming a capsule or slime layer
• H antigen: The protein peritrichous flagella on motile strains
• Many Enterobacteriaceae have antigenic surface pili (fimbriae)

Helicobacter pylori

• Spiral bacilli that are human gastric pathogens, swim in mucus layer of stomach
• Protected from acid by ammonia from hydrolysis of urea, causing chronic gastritis, gastric ulcers, and possibly gastric carcinomas
• Resides for long periods of time with or without clinical symptoms