[RECOMMENDED] Review of Medical Microbiology and Immunology, 16th Edition

Lysozyme degrades peptidoglycan.

Bacterial spores are metabolically inactive, yet can survive for years in that inactive state.

The three basic shapes of bacteria are cocci (round), bacilli (rods), and spirochetes (spiral-shaped).

Cocci can occur in pairs (diplococci), chains (streptococci), or grapelike clusters (staphylococci), depending on their orientation and degree of attachment during cell division.

Peptidoglycan provides structural support and maintains the characteristic shape of the bacterial cell.

Mycoplasma species are bounded by a cell membrane instead of a cell wall, which is common to all other bacteria.

Bacteria range in size from about 0.2 to 5 µm. The smallest bacteria, like Mycoplasma, are comparable in size to the largest viruses, while the longest bacteria can be as large as some yeasts and human red blood cells (7 µm).

The peptidoglycan layer is much thicker in gram-positive bacteria compared to gram-negative bacteria.

The gram-negative bacterial envelope consists of:

1. Lipopolysaccharide (LPS)
2. Lipoprotein
3. Phospholipid

Additionally, it includes the periplasmic space which may contain enzymes like β-lactamases.

Teichoic acids are fibers that protrude outside the peptidoglycan layer in gram-positive bacteria, contributing to the structural integrity and function of the cell wall.

The periplasmic space in gram-negative bacteria is significant as it may contain enzymes such as β-lactamases that degrade penicillins and other β-lactam drugs.

Endotoxins are components of lipopolysaccharides (LPS) found in the envelope of gram-negative bacteria, which can trigger immune responses in hosts.

Bacteria generally range in size from Mycoplasma, the smallest, to Bacillus anthracis, one of the largest, while viruses like poliovirus are smaller than bacteria, and protozoa are typically larger than bacteria.

The bacterial cell wall provides rigid support, protects against osmotic pressure, serves as the site of action for penicillins and cephalosporins, and is degraded by lysozyme.

Peptidoglycan consists of a glycan (sugar) backbone with peptide side chains that are cross-linked, providing structural integrity to the bacterial cell wall.

Gram-positive bacteria have a thicker and multilayered peptidoglycan layer, while gram-negative bacteria have a thinner peptidoglycan layer with fewer layers.

Porin proteins facilitate the passage of small, hydrophilic molecules into the cell, allowing entry of essential substances such as sugars, amino acids, vitamins, and metals, as well as many antimicrobial drugs like penicillins.

Mycolic acids contribute to the acid-fast property of mycobacteria, making them resistant to decolorization with acid-alcohol after staining, which is crucial for their identification in laboratory settings.

1. Attachment pili: Mediate attachment to cell surfaces.
2. Sex pili: Mediate attachment of two bacteria during conjugation.

The capsule, composed of polysaccharides, protects bacteria against phagocytosis, enhancing their virulence.

The plasma membrane, a lipoprotein bilayer without sterols, is the site of oxidative and transport enzymes, playing a crucial role in cellular metabolism.

Ribosomes, composed of RNA and protein, are responsible for protein synthesis and are the site of action for various antibiotics such as aminoglycosides, erythromycin, tetracyclines, and chloramphenicol.

Plasmids are small, circular DNA molecules that can carry genes for antibiotic resistance and toxins, contributing to bacterial adaptability and survival.

• Gram-positive bacteria have a thicker peptidoglycan layer and contain teichoic acids. They lack an outer membrane.
• Gram-negative bacteria have a thinner peptidoglycan layer, possess an outer membrane that contains lipopolysaccharides (LPS), and have a periplasmic space where β-lactamases are found.

The cell wall of Mycobacterium tuberculosis contains:

• Mycolic acid
• Arabinoglycan
• Peptidoglycan These components are not typically found in most other genera of bacteria.

Peptidoglycan is composed of:

• A glycan chain made of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).
• A tetrapeptide chain of amino acids.
• A peptide interbridge that connects the tetrapeptide chains in Gram-positive cells. This structure forms a multilayered, three-dimensional network in the cell wall.

1. The crystal violet dye stains all cells purple.

2. The iodine solution (a mordant) is added to form a crystal violet-iodine complex; all cells continue to appear purple.

3. The organic solvent extracts the purple dye complex from gram-negative bacteria more than from gram-positive bacteria, making gram-negative appear colorless.

4. The red dye safranin stains the decolorized gram-negative cells red/pink, while gram-positive bacteria remain purple.

Gram-positive bacteria are generally more susceptible to penicillin G than gram-negative bacteria, which influences the choice of antibiotic treatment.

Peptidoglycan is present in bacteria but not in human cells, making it a good target for antibacterial drugs. Drugs like penicillins, cephalosporins, and vancomycin inhibit its synthesis by targeting the transpeptidase enzyme responsible for cross-linking tetrapeptides.

Lysozyme can cleave the peptidoglycan backbone, contributing to the natural resistance of the host to microbial infection. It can cause bacteria to swell and rupture due to osmotic pressure, but if in isotonic solution, they survive as protoplasts.

LPS is an endotoxin found in the outer membrane of gram-negative bacteria, responsible for disease features such as fever and shock, particularly hypotension.

1. Lipid A - a phospholipid responsible for toxic effects.

2. Core polysaccharide - composed of five sugars linked to lipid A through keto-deoxyoctulonate (KDO).

3. Outer polysaccharide - consists of up to 25 repeating units of three to five sugars, serving as the somatic or O antigen for identification of certain gram-negative bacteria.

Teichoic acids serve several important functions:

1. Induce inflammation - they can activate pathways similar to endotoxins in gram-negative bacteria, leading to septic shock.

2. Mediating attachment - they help staphylococci attach to mucosal cells.

3. Structural support - they provide rigidity to the cell wall by protruding from the peptidoglycan layer.

The cytoplasmic membrane has the following functions:

1. Active transport of molecules into the cell.

2. Energy generation through oxidative phosphorylation.

3. Synthesis of precursors for the cell wall.

4. Secretion of enzymes and toxins.

Bacterial ribosomes are:

• Size: 70S (composed of 50S and 30S subunits) compared to eukaryotic ribosomes which are 80S (composed of 60S and 40S subunits).
• Chemical composition: Differences in ribosomal RNAs and proteins allow for selective action of antibiotics that inhibit bacterial protein synthesis without affecting human cells.

The nucleoid contains the following:

• DNA: Typically a single, circular molecule, though exceptions exist (e.g., Vibrio cholerae has two circular chromosomes).
• No nuclear membrane: Unlike eukaryotic cells, the nucleoid lacks a nuclear membrane, nucleolus, mitotic spindle, and histones.

Metachromatic granules serve as storage areas for nutrients, specifically:

• Volutin: A reserve of high energy stored as polymerized metaphosphate.
• Staining characteristics: They stain red with methylene blue dye, which is a characteristic feature of Corynebacterium diphtheriae, the causative agent of diphtheria.

Plasmids are extrachromosomal, double-stranded, circular DNA molecules that replicate independently of the bacterial chromosome. They can be integrated into the bacterial chromosome and exist in both gram-positive and gram-negative bacteria.

1. Transmissible plasmids: Large (MW 40-100 million), contain genes for sex pilus synthesis and transfer enzymes, usually present in 1-3 copies per cell.

2. Nontransmissible plasmids: Small (MW 3-20 million), do not contain transfer genes, frequently present in 10-60 copies per cell.

Plasmids carry genes for:

1. Antibiotic resistance (e.g., β-lactamase)
2. Exotoxins (e.g., enterotoxins, anthrax toxin)
3. Pili (fimbriae) for adherence to epithelial cells
4. Resistance to heavy metals (e.g., mercury)
5. Resistance to ultraviolet light (DNA repair enzymes)
6. Bacteriocins (toxic proteins against other bacteria).

Transposons are pieces of DNA that can move within or between the DNAs of bacteria, plasmids, and bacteriophages. They can carry drug-resistant genes, toxins, or metabolic enzymes, and can cause mutations or alter gene expression upon insertion.

Transposons typically have four identifiable domains:

1. Inverted repeats: Short DNA sequences at each end involved in integration.
2. Transposase gene: Enzyme that mediates excision and integration.
3. Repressor gene: Regulates synthesis of transposase and other proteins.
4. Antibiotic resistance gene: Often encodes an enzyme mediating resistance.

The capsule is a gelatinous layer that covers the bacterium, composed of polysaccharides. It is important for:

1. Virulence: Limits phagocyte engulfment.
2. Serotyping: Determines serologic type based on sugar composition.
3. Protection: Provides a defense against environmental hazards.
4. Adherence: Aids in bacterial adherence to surfaces.

The Quellung reaction is a swelling phenomenon used to identify certain organisms by using antiserum against the capsular polysaccharide. In the presence of homologous antibody, the capsule swells greatly, indicating the presence of the organism.

Capsular polysaccharides are used as antigens in certain vaccines because they can elicit protective antibodies. For example, the capsular polysaccharides of S. pneumoniae, Neisseria meningitidis, and Haemophilus influenzae are immunogens in vaccines against these bacteria.

Flagella are long, whip-like appendages that enable bacteria to move toward nutrients through a process called chemotaxis. They play a role in pathogenesis by propelling bacteria, such as E. coli, up the urethra into the bladder, contributing to urinary tract infections.

Pili mediate the attachment of bacteria to specific receptors on human cell surfaces, which is crucial for initiating infection. Additionally, the sex pilus facilitates the attachment between donor and recipient bacteria during conjugation.

The glycocalyx is a polysaccharide coating that allows bacteria to adhere firmly to various structures, playing a key role in biofilm formation. It is significant in infections, as glycocalyx-producing strains of bacteria like P. aeruginosa and Staphylococcus epidermidis are associated with respiratory infections and endocarditis, respectively.

Bacterial spores are highly resistant structures formed by genera like Bacillus and Clostridium in response to adverse conditions. They contain bacterial DNA, a small amount of cytoplasm, and a keratin-like coat that provides resistance to heat, dehydration, radiation, and chemicals, potentially mediated by dipicolinic acid.

• Bacteria have three shapes: cocci (spheres), bacilli (rods), and spirochetes (spirals).
• Cocci are arranged in three patterns: pairs (diplococci), chains (streptococci), and clusters (staphylococci).
• The size of most bacteria ranges from 1 to 3 µm. Mycoplasma, the smallest bacteria, are 0.2 µm. Some bacteria, such as Borrelia, can be as long as 10 µm.

• Peptidoglycan is a network that covers the entire bacterium and gives the organism its shape.
• It is composed of a sugar backbone (glycan) and peptide side chains (peptido).
• The side chains are cross-linked by transpeptidase, the enzyme inhibited by penicillins and cephalosporins.

Lysozymes kill bacteria by cleaving the glycan backbone of peptidoglycan.

Gram-positive bacteria stain purple due to their ability to retain the crystal violet-iodine complex, while gram-negative bacteria stain pink because they lose the purple dye when treated with acetone-alcohol.

Capsules are antiphagocytic, limiting the ability of neutrophils to engulf bacteria. They are composed mostly of polysaccharide and serve as antigens in vaccines, enhancing the effectiveness of antibodies in phagocytosis.

Plasmids are extrachromosomal pieces of circular DNA that encode exotoxins and many enzymes responsible for antibiotic resistance.

Bacterial spores are highly heat resistant and not killed by many disinfectants, making them significant in infection control. They can survive for many years and germinate into pathogenic bacteria when conditions are favorable.

Pili are filaments of protein that extend from the bacterial surface, mediating the attachment of bacteria to human cells. They also include a type called sex pilus, which is involved in conjugation.

The purpose of using alcohol in the Gram stain procedure is to disrupt the outer cell membrane of gram-negative bacteria, allowing the purple dye to leave the cells, which results in them becoming colorless before staining pink with safranin.

Mycobacterium tuberculosis does not stain well with the Gram stain because it has a large amount of lipid in its cell wall, which prevents the entry of the purple dye, making it stain well with the acid-fast stain instead.

The capsule exhibits the most antigenic variation among bacterial components.

B-lactamases are most commonly located within the periplasmic space.

Gram-positive bacteria have a thick peptidoglycan cell wall containing teichoic acids and lack an outer membrane. Gram-negative bacteria have a thin peptidoglycan layer located in the periplasmic space plus an outer membrane that contains lipopolysaccharide (LPS).

The extracellular substance is called glycocalyx.