Infectious Diseases: Microbes & Features

Microbes Causing Infectious Diseases

• Agents of human infectious diseases are categorized into bacteria, fungi, protozoa, helminths, and viruses.
• Bacteria belong to the Bacteria domain.
• Fungi (yeasts and molds), protozoa, and helminths (worms) are in the Eukarya domain.
• Bacteria, fungi, and protozoa are unicellular or simple multicellular.
• Helminths are complex multicellular.
• Viruses are noncellular and distinct from other organisms.

Important Microbial Features

• Cells contain a nucleus or nucleoid with DNA, surrounded by cytoplasm for protein synthesis and energy generation.
• Viruses have a genetic material core (DNA or RNA) but lack cytoplasm, relying on host cells for protein synthesis and energy.
• Cells replicate through binary fission or mitosis, producing progeny cells that maintain the cellular structure.
• Bacteria replicate by binary fission, while eukaryotic cells use mitosis.
• Viruses disassemble, replicate nucleic acid and protein, and reassemble into new viruses, requiring host cells due to lacking protein synthesis and energy systems.
• Rickettsiae and chlamydiae also need living host cells for growth.
• Cells contain both DNA and RNA, but viruses have either DNA or RNA, not both.

Comparison of Eukaryotes and Bacteria

• Eukaryotes (fungi, protozoa, helminths) differ from bacteria in structure and complexity.
• Fungi, protozoa, and helminths possess a true nucleus with multiple chromosomes and a nuclear membrane and use mitotic apparatus.
• Bacterial nucleoid consists of a single circular DNA molecule without a nuclear membrane or mitotic apparatus.
• Eukaryotic cells have organelles (mitochondria and lysosomes) and larger (80S) ribosomes, while bacteria lack organelles and have smaller (70S) ribosomes.
• Most bacteria have a peptidoglycan cell wall, but eukaryotes do not.
• Fungi have a rigid chitin cell wall.
• Eukaryotic cell membranes contain sterols; prokaryotes (except Mycoplasma) do not.
• Motility varies; most protozoa and some bacteria are motile, while fungi and viruses are nonmotile.
• Protozoa use flagella, cilia, and pseudopods for movement.
• Bacteria move solely via flagella.

Terminology

• Bacteria, fungi, protozoa, and helminths are named using the binomial Linnaean system (genus and species).
• Escherichia coli (Escherichia = genus, coli = species).
• Candida albicans (Candida = genus, albicans = species).
• Viruses usually have a single name.
• Poliovirus, measles virus, or rabies virus.
• Some viruses have two-word names (herpes simplex virus) but do not denote genus and species.

Shape and Size of Bacteria

• Bacteria shapes are classified as cocci (round), bacilli (rods), or spirochetes (spiral).
• Some bacteria can be pleomorphic, exhibiting variable shapes.
• A bacterium's shape is determined by its rigid cell wall.
• Microscopic appearance is an important identification criterion.
• Bacteria arrangements are also important: cocci in pairs (diplococci), chains (streptococci), or clusters (staphylococci).
• Bacteria size ranges from 0.2 to 5 µm.
• Mycoplasma (smallest bacteria) are about the same size as poxviruses (largest viruses).
• Longest bacteria are around the size of some yeasts and human red blood cells (7 µm).

Bacterial Cell Wall

• The cell wall is an outermost component for most bacteria (except Mycoplasma).
• Some have external surface features like capsules, flagella, and pili.
• The cell wall is external to the cytoplasmic membrane and is composed of peptidoglycan.
• Peptidoglycan provides structural support and maintains cell shape.

Gram-Positive and Gram-Negative Bacteria

• Cell wall structure, chemical composition, and thickness differ between gram-positive and gram-negative bacteria.
• Gram-positive bacteria have thicker peptidoglycan layers.
• Many gram-positive bacteria have teichoic acid fibers protruding outside the peptidoglycan.
• Gram-negative bacteria have a complex outer layer of lipopolysaccharide (LPS), lipoprotein, and phospholipid, referred to as the "envelope".
• Gram-negative bacteria have a periplasmic space between the outer membrane and cytoplasmic membrane.
• Periplasmic space contains β-lactamases that degrade penicillins.

Gram Stain Procedure

• Gram stain differentiates bacteria: gram-positive (purple) and gram-negative (red).
• Crystal violet stains all cells purple.
• Iodine is added to form a crystal violet-iodine complex.
• Organic solvent extracts the dye/iodine from lipid-rich, thin-walled, gram-negative bacteria, which appear colorless.
• Safranin stains decolorized gram-negative cells red/pink; gram-positive bacteria remain purple.
• Gram stain helps identify bacteria and choose antibiotics.
• Gram-positive bacteria are more susceptible to penicillin G.
• Some bacteria (Mycobacteria, Treponema, Mycoplasma, Legionella, Chlamydiae, and Rickettsiae) aren't seen in Gram stain.
• Neisseria meningitidis can be directly observed in a Gram stain of blood due to its high concentration in the blood.

Properties of the Cell Wall

• Gram-negative bacteria envelopes contain endotoxin, an LPS.
• Both gram-positive and gram-negative bacteria have surface polysaccharides and proteins that act as antigens for identification.
• Porin proteins facilitate the passage of hydrophilic molecules across the outer membrane of gram-negative bacteria and are found in gram-positive bacteria.

Cell Walls of Acid-Fast Bacteria

• Mycobacteria have unusual cell walls that prevent Gram staining; are acid-fast i.e. resist decolorization with acid-alcohol.
• High lipid concentration of mycolic acids is why mycobacteria are acid-fast.
• Nocardia asteroides, is weakly acid-fast, meaning decolorization depends on the strength of hydrochloric acid in the staining process

Peptidoglycan

• Peptidoglycan is a complex network that surrounds cell and provides support.
• Found only in bacterial cell walls; maintains cell shape and resistance to osmotic pressure.
• Peptidoglycan consists of glycan chains (NAM and NAG), a tetrapeptide chain, and a peptide interbridge.
• Glycan chain contain alternating N-acetylmuramic acid and N-acetylglucosamine molecules.
• Diaminopimelic acid and D-alanine are unique to bacterial cell walls and are targeted by penicillin.
• Cross-links vary among species; Staphylococcus aureus uses five glycines to link D-alanine to L-lysine.
• Antibacterial drugs (penicillins, cephalosporins, vancomycin) inhibit peptidoglycan synthesis.
• Lysozyme cleaves peptidoglycan backbone, contributing to natural resistance.

Lipopolysaccharide (LPS)

• LPS (endotoxin) is in the outer membrane of gram-negative bacteria cell walls, causing fever and shock (hypotension).
• LPS consists of lipid A (responsible for toxicity), a core polysaccharide, and an outer polysaccharide.
• The outer polysaccharide is a somatic (O) antigen for identifying organisms.
• Some bacteria have lipooligosaccharide (LOS) instead.

Teichoic Acid

• Teichoic acids/lipoteichoic acids are anchored to the cell wall/membrane and cause inflammation/septic shock from gram-positive bacteria and activate similar pathways to endotoxin.
• Also mediate staphylococci attachment to mucosal cells.
• Gram-negative bacteria lack teichoic acids.

Cytoplasmic Membrane

• The cytoplasmic membrane is a phospholipid bilayer inside the peptidoglycan layer composed of a phospholipid bilayer similar to that of eukaryotic cells.
• Eukaryotic membranes possess sterols, which prokaryotes generally lack, with the exception of Mycoplasma.
• Active transport of molecules.
• Energy generation via oxidative phosphorylation.
• Synthesis of cell wall precursors.
• Secretion of enzymes and toxins.

Cytoplasm

• Amorphous matrix that contains ribosomes, nutrient granules, metabolites, and plasmids.
• Nucleoid region composed of DNA.

Ribosomes

• Bacterial ribosomes are the site of protein synthesis as in eukaryotic cells, but differ from eukaryotic ribosomes in size and chemical composition.
• Bacterial ribosomes are 70S (50S and 30S subunits) compared to eukaryotic 80S ribosomes.
• Differences in ribosomal RNA and proteins allow antibiotics to inhibit bacterial protein synthesis.

Nucleoid

• The nucleoid houses DNA.
• Most prokaryotes have single, circular DNA molecules, w/ some having a genome of two chromosomes.
• Borrelia burgdorferi, the spirochete that causes Lyme disease, is composed of a linear chromosome and multiple circular and linear plasmids.
• The size of bacterial genomes varies widely, with the smallest genome containing just over 130 genes and the largest containing approximately 11,600 genes.
• Human DNA has approximately 25,000 genes.
• Bacterial nucleoid lacks nuclear membrane, nucleolus, mitotic spindle, histones, and introns.

Plasmids

• Double-stranded, circular DNA molecules capable of replicating independently of bacterial chromosome.
• Usually extrachromosomal, but can integrate into the bacterial chromosome.
• Found in both gram-positive and gram-negative bacteria.
• Transmissible plasmids transfer from cell to cell by conjugation and contain genes for sex pilus synthesis and transfer enzymes (MW 40–100 million; 1-3 copies per cell).
• Nontransmissible plasmids are small and lack transfer genes (MW 3-20 million; 10-60 copies per cell).
• Antibiotic resistance (mediated by enzymes).
• Exotoxins like E. coli enterotoxins, Bacillus anthracis anthrax toxin, staphylococcal exfoliative toxin, and Clostridium tetanus toxin

Transposons

• DNA pieces that move readily from one site to another within or between DNAs of bacteria, plasmids, and bacteriophages
• Also known as “jumping genes.”
• Transposons typically contain a gene for transposase that mediates process as well as a gene for a repressor and a gene mediating antibiotic resistance.

Capsule

• The capsule is a gelatinous layer that covers the entire bacterium, typically composed of polysaccharide.
• Capsule prevents phagocytes from engulfing bacteria.
• Specific identification of an organism is made by using antiserum against the capsular polysaccharide (Quellung reaction).
• Capsular polysaccharides act as antigens in vaccines.
• Capsules aid in adherence to human tissues.

Flagella

• Flagella are whip-like appendages for chemotaxis.
• Flagella are composed of subunits of flagellin, arranged in intertwined chains.
• Flagellated bacteria have defined flagella numbers and locations.
• Spirochetes use an axial filament for movement.
• Flagella aid in urinary tract infections.

Pili (Fimbriae)

• Pili are hair-like filaments extending from the cell surface made up of pilin subunits.
• Pili mediate attachment to cell surface receptors.
• A specialized pilus, sex pilus, facilitates conjugation.

Glycocalyx

• Glycocalyx is a polysaccharide coating secreted by many bacteria to help adhere to surfaces like skin, heart valves, and catheters.
• Glycocalyx-producing strains are responsible for cystic fibrosis infections, endocarditis, and plaque formation on teeth.

Bacterial Spores

• Highly resistant structures formed by bacteria when nutrients are depleted.
• Formed by Bacillus and Clostridium species and contain bacterial DNA, cytoplasm, cell membrane, peptidoglycan, very little water, and a thick, keratin-like coat.
• Contain dipicolinic acid, a calcium ion chelator spore.
• Spores are resistant to heat and chemicals; steam heating under pressure at 121°C is required for effective sterilization.

Bacterial Growth Cycle

• Number of bacteria is exponential.
• Doubling (generation) time varies by species (20 minutes for Escherichia coli to as long as 18 to 24 hours for Mycobacterium tuberculosis).
• Lag, log (logarithmic or exponential), stationary, and death phases.

Obligate Intracellular Growth

• Obligate intracellular pathogens (Chlamydia and Rickettsia and Ehrlichia and Anaplasma) can only grow within living cells because they lack the ability to produce sufficient adenosine triphosphate (ATP) and must use ATP produced by the host cells.

Aerobic and Anaerobic Growth

• Adequete supply of oxygen enhances metabolism and growth for most organisms.
• The oxygen acts as the hydrogen acceptor in the final steps of energy production catalyzed by the flavoproteins and cytochromes.
• Toxic molecules such as superoxide dismutase that convert 02 to H2O2, followed by catalase, which reduces H2O2 to harmless water and oxygen molecules.

Fermentation of Sugars

• The breakdown of a sugar (such as glucose or maltose or lactose) to pyruvic acid and then, usually, to lactic acid is usually caused by fermentation.
• Facultative bacteria is able to break down sugars regardless, if oxygen is present or not.

Iron Metabolism

• Iron, in the form of ferric ion, is required for the growth of bacteria because it is an essential component of cytochromes and other enzymes.
• Bacteria produce iron-binding compounds called siderophores to obtain iron that facilitate access for pathogenic bacteria.

Bacterial Genetics and Mutations

• Bacteria are haploid, usually possessing a single chromosome and therefore a single copy of each gene. Mutations can occur by:
• Base substation
• Frameshift mutation
• Transposons and insertion sequences. Mutations are caused by:
• Chemicals
• Radiation
• Viruses

Conditional lethal mutations

• Conditional lethal mutations are of medical interest because they may be useful in vaccines (e.g., influenza vaccine).

Transfer of DNA Within Bacterial Cells

• Transposons transfer DNA from one site within the bacterial genome to another site.
• Transfer of DNA within bacteria also occurs by programmed rearrangements.

Transfer of DNA Between Bacterial Cells

The transfer of genetic information from one cell to another can occur by three methods:

• Conjugation
• Transduction
• Transformation

Recombination

There are two types of recombination:

• Homologous recombination
• Nonhomologous recombination:

Classification of Bacteria

The initial criterion used in the classification is the nature of the cell wall (i.e., is it rigid, flexible, or absent?).

• rigid, thick-walled cells.
• flexible, thin-walled cells (spirochetes)
• Those without cell walls (the mycoplasmas)

The Human Microbiome

• Human microbiome is the term used to describe the distinct microbial communities that inhabit different host environments on the body's skin and mucosal surfaces, a necessary and normal part of human development.

Colonization resistance

• Normal flora occupy receptor sites on the skin and mucosal surfaces, thereby preventing pathogens from binding to those receptors.