Introduction to infection and bacteria

Question 1

Why are infectious diseases important?

Answer 1

Infectious diseases cause significant morbidity and mortality in both resource-poor and resource-rich countries. They have played a crucial role throughout human history and evolution.

Question 2

What are infections acquired in hospitals called?

Answer 2

Infections acquired in hospitals are known as nosocomial infections.

Question 3

Name some examples of nosocomial infections.

Answer 3

Examples of nosocomial infections include Staphylococcus aureus (including MRSA), Enterococcus species (including VRE), Clostridium difficile, nosocomial pneumonia, and wound infections. Implant and device-related infections can also occur.

Question 4

What is the human microbiome?

Answer 4

The human microbiome refers to the genetic material of all the microbes (bacteria, fungi, protozoa, and viruses) that live on and inside the human body.

Question 5

How does the number of genes in a person’s microbiome compare to the number of genes in the human genome?

Answer 5

The number of genes in all the microbes in a person’s microbiome is 200 times greater than the number of genes in the human genome.

Question 6

What are some diseases associated with an altered microbiome?

Answer 6

Diseases associated with an altered microbiome include obesity, inflammatory bowel disease (IBD), liver diseases such as cirrhosis, alcoholic liver disease (ALD), and non-alcoholic fatty liver disease (NAFLD). Other diseases linked to the microbiome include emerging infectious diseases, diabetes mellitus, atherosclerosis, and metabolic syndrome.

Question 7

How do we link microorganisms to disease?

Answer 7

Microorganisms can be linked to disease through Koch’s postulates. These postulates state that a specific microorganism is always associated with a given disease, the microorganism can be isolated from the diseased individual and grown in pure culture in the laboratory, the cultured microbe will cause disease when transferred to a healthy individual, and the same type of microorganism can be isolated from the newly infected individual.

Question 8

What are the different types of microorganisms?

Answer 8

The different types of microorganisms include viruses, prokaryotes (bacteria and archaea), and eukaryotes (fungi and protists).

Question 9

What are the key features of the structure of microbes?

Answer 9

The key features of the structure of microbes include the boundary, which acts as a barrier from the environment, the presence of a cell wall (in some microbes) and a membrane, the cytoplasm, which is an aqueous mixture of macromolecules such as proteins, lipids, nucleic acids, polysaccharides, and other organic and inorganic molecules. Some microbes also have organelles. Membrane permeability and mechanisms of transport are essential for nutrient intake and waste removal.

Question 10

How are microbes named in taxonomy?

Answer 10

Microbes are named in taxonomy using scientific names that consist of two parts. The scientific names are written in italics or underlined. For example:

Homo sapiens (human)
Staphylococcus aureus
Streptococcus pyogenes
Escherichia coli

Question 11

What are the different types of microorganisms?

Answer 11

The different types of microorganisms include:

Viruses
Prokaryotes (bacteria and archaea)
Eukaryotes (fungi and protists)

Question 12

What are prokaryotes?

Answer 12

Prokaryotes are simple, unicellular organisms that lack a defined nucleus, mitochondria, or other membrane-bound organelles. They include archaea and bacteria and have rapid reproduction rates. While most prokaryotes are highly beneficial, providing protection to skin/epithelial tissue and aiding in food digestion, some can be pathogenic.

Question 13

Give examples of prokaryotes.

Answer 13

Examples of prokaryotes include archaea and bacteria.

Question 14

What are fungal pathogens?

Answer 14

Fungal pathogens are eukaryotes that can cause diseases. Examples include:

Candida species (e.g., Candida albicans): A yeast that is unicellular and reproduces by budding.
Aspergillus species (e.g., Aspergillus fumigatus): A mold that is multicellular and reproduces by spores.

Question 15

What are some examples of protozoa and the diseases they cause?

Answer 15

Some examples of protozoa and the diseases they cause include:

Plasmodium falciparum: It is the malaria parasite responsible for causing malaria.
Giardia lamblia: It is a protozoan that causes Giardiasis, a gastrointestinal infection.
Entamoeba histolytica: It is the causative agent of amoebic dysentery, an intestinal infection.

Question 16

What are some examples of helminths?

Answer 16

Some examples of helminths, which are parasitic worms, include:

Taenia saginata: It is a beef tapeworm that infects humans through the consumption of undercooked beef.
Loa loa: It is commonly known as the African eyeworm and can cause a disease called loiasis.

Question 17

Why is bacterial classification important?

Answer 17

Bacterial classification is important because it allows us to distinguish and categorize medically important bacteria. Different bacteria have distinct clinical presentations, diagnostic methods, and treatment strategies. Therefore, proper classification is crucial for understanding and managing bacterial infections effectively.

Question 18

How are bacteria classified based on their morphology?

Answer 18

Bacteria are commonly classified based on their shape and staining characteristics. Some common classifications include:

Round-shaped bacteria: Known as “cocci” or “coccal” bacteria. Examples include Streptococcus species and Enterococcus species.
Long-shaped bacteria: Referred to as “bacilli” or “bacillus” bacteria. Examples include Enterobacter species.
Some bacteria exhibit spiral or branched (filamentous) shapes, as well as comma-shaped forms.

Question 19

What is the basis for Gram-positive and Gram-negative bacterial classification?

Answer 19

The classification of bacteria into Gram-positive and Gram-negative groups is based on their ability to take up stain due to the differences in the thickness and accessibility of their cell wall peptidoglycans.

Question 20

How are Gram-positive and Gram-negative bacteria differentiated using Gram staining?

Answer 20

Gram staining protocol involves several steps:

Bacteria are dried on a glass plate.
They are then stained with crystal violet and set with iodine.
Decolorization is carried out using alcohol or acetone.
Counterstaining is done using safranin, resulting in a pink color for Gram-negative bacteria and a purple color for Gram-positive bacteria.

Question 21

Can you provide examples of bacterial morphology and staining?

Answer 21

Examples of bacterial morphology and staining include:

Gram-positive cocci: Examples include Staphylococcus and Streptococcus species.
Gram-negative cocci: Examples include Neisseria species.
Gram-negative bacilli: Examples include Escherichia coli and Pseudomonas aeruginosa.
Gram-positive bacilli: Examples include Bacillus and Clostridium species.

Question 22

Could you provide some examples of medically important bacteria?

Answer 22

Some examples of medically important bacteria include Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative), Streptococcus pneumoniae (Gram-positive), and Pseudomonas aeruginosa (Gram-negative).

Question 23

What is the structure of peptidoglycan?

Answer 23

Peptidoglycan is a three-dimensional polymer consisting of N-acetylated sugars, namely N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). It also includes 3-5 amino acid peptides that are unique to peptidoglycans and are resistant to enzymatic destruction. The peptidoglycan structure is cross-linked by transpeptidase enzymes.

Question 24

How is peptidoglycan synthesized?

Answer 24

Peptidoglycan synthesis involves unique pathways found in bacteria. The synthesis includes the following steps:

Polymerization of sugars: NAG and NAM are assembled to form the backbone of peptidoglycan.
Elongation of amino acid side-chains: Peptides are added to the peptidoglycan structure.
Transpeptidase: Transpeptidase enzymes cross-link the peptidoglycan strands.
These steps involved in peptidoglycan synthesis are targeted by important classes of antibiotics.

Question 25

How are mycobacteria different from other bacteria?

Answer 25

Mycobacteria have some unique characteristics that differentiate them from other bacteria:

They have a Gram-positive cell wall structure but do not stain Gram-positive due to the presence of a very thick lipid membrane called the mycomembrane, which is anchored to the peptidoglycan layer.
They have the ability to survive and replicate inside host cells, contributing to their intracellular survival.
Mycobacterium tuberculosis complex includes species such as M. tuberculosis, M. bovis, M. africanum, M. microti, and others. Non-tuberculous mycobacteria, also known as ‘atypical’ mycobacteria, include species such as M. ulcerans, M. avium-intracellulare, M. kansasii, M. chelonae, etc.
Mycobacterium leprae is the causative agent of leprosy.

Question 26

What are some examples of pathogenic mycobacteria and ‘atypical’ mycobacteria?

Answer 26

Examples of pathogenic mycobacteria include species within the Mycobacterium tuberculosis complex, which causes tuberculosis (TB). ‘Atypical’ mycobacteria, or non-tuberculous mycobacteria, encompass various species such as M. ulcerans, M. avium-intracellulare, M. kansasii, M. chelonae, among others.

Question 27

How do bacteria grow and form clusters or chains?

Answer 27

Bacteria can exhibit different growth patterns. Some bacteria may grow in clusters or chains, which is especially notable for Gram-positive cocci such as Staphylococci and Streptococci. These bacteria have specific mechanisms of division and cell wall synthesis that result in their characteristic clustering or chain formation.

Question 28

How do bacteria grow and what can be exploited for infection diagnosis?

Answer 28

Bacteria can exhibit different growth patterns, forming chains or clusters under microscopic view and colonies of different sizes and shapes on solid culture medium. During growth, bacteria excrete enzymes and waste products into the environment. Bacterial growth has different requirements, including atmospheric conditions and nutrient availability. These growth characteristics and requirements can be exploited for diagnosing bacterial infections.

Question 29

What are the atmospheric requirements of bacteria?

Answer 29

Bacteria can have different atmospheric requirements:

Aerobes: They require oxygen and use it as the final electron acceptor in their metabolic processes, such as the oxidation of glucose to CO2 and H2O.
Anaerobes (obligate anaerobes): They cannot tolerate oxygen and use alternative electron acceptors, often organic molecules, through fermentation. For example, glucose can be fermented to lactic acid.
Facultative anaerobes: They can switch between aerobic and anaerobic metabolism depending on the availability of oxygen.

Question 30

What are the nutritional requirements of bacteria?

Answer 30

Bacteria have various nutritional requirements, and they need to import essential components they cannot synthesize themselves. This includes purines, pyrimidines, amino acids, and vitamins. The nutritional requirements can vary among different bacterial species. For example, Escherichia coli (E. coli) can grow with glucose and inorganic salts, making it relatively easy to cultivate in the laboratory. On the other hand, bacteria like Treponema pallidum, the causative agent of syphilis, have specialized enriched media requirements, making their growth more challenging.

Question 31

What are some physical growth requirements for bacteria?

Answer 31

Bacteria have specific physical growth requirements, including:

Temperature: Bacteria have optimal growth temperatures, such as mesophiles (moderate temperature), thermophiles (high temperature), and psychrophiles (low temperature).
pH: Bacteria have different pH preferences for growth, ranging from acidophiles (acidic pH) to alkaliphiles (alkaline pH).
Salt content: Some bacteria require specific salt concentrations for growth, and they are categorized as halophiles (salt-loving) or halotolerant (able to tolerate high salt concentrations).

Question 32

What is exponential growth and the typical growth curve in liquid culture?

Answer 32

Exponential growth refers to a phase of bacterial growth where the population size doubles at a constant rate. The typical growth curve in liquid culture consists of the following phases:

Lag phase: In this phase, there is no significant increase in cell numbers as bacteria adjust to a new environment, undergo gene regulation, and prepare for active growth.
Exponential phase: Also known as the log phase, this phase is characterized by rapid cell division and exponential increase in cell numbers. The slope of the curve during this phase represents the growth rate of the organism in that specific environment.
Stationary phase: Nutrient depletion and accumulation of metabolites lead to the cessation of cell division. The population size remains relatively constant, and gene regulation plays a role in adapting to the changing conditions.
Death phase: Resources are exhausted, and the toxic environment causes a decline in cell viability and a decrease in population size.

Question 33

What is the importance of bacterial envelope structure?

Answer 33

The bacterial envelope structure plays several important roles:

Determines gram staining: The presence or absence of a thick peptidoglycan layer in the cell wall influences whether a bacterium stains as Gram-positive or Gram-negative, which in turn provides information about its characteristics.
Influences susceptibility to antibiotics: Differences in the structure and composition of the bacterial envelope can affect the susceptibility of bacteria to various antibiotics.
Determines pathogenicity: The components of the bacterial envelope, such as lipopolysaccharide (endotoxin) found in Gram-negative bacteria, contribute to their pathogenic potential and ability to cause disease.
Presence of endotoxin: Lipopolysaccharide (endotoxin) is found only in Gram-negative bacteria and can elicit a strong immune response in host organisms.
Differences in exotoxins: Exotoxins, which are toxins produced and secreted by bacteria, can be found in both Gram-positive and Gram-negative bacteria and contribute to their virulence.

Question 34

What are the components of endotoxin in Gram-negative bacteria?

Answer 34

The components of endotoxin in Gram-negative bacteria include:

O antigen: It is a repeating glycan polymer that varies based on the bacterial strain. It is immunogenic and contributes to the diversity of Gram-negative bacteria.
Lipid A: It serves as an anchor for lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. Lipid A is highly immunogenic and can elicit strong immune responses.

Question 35

What is the function of a bacterial capsule?

Answer 35

The capsule is a polysaccharide coat that surrounds the bacterial cell wall. It serves multiple functions, including:

“Hiding” the immunogenic cell wall and making bacteria less recognizable to the immune system.
Conferring virulence by protecting bacteria from host immune responses, such as phagocytosis.
Acting as an adhesin, allowing bacteria to adhere to surfaces and form biofilms.
Playing a role in evading the immune response and promoting bacterial survival within the host.

Question 36

What are ribosomes and their role in protein synthesis?

Answer 36

Ribosomes are cellular structures responsible for protein synthesis. In bacteria, ribosomes are 70S in size, smaller than the 80S ribosomes found in eukaryotes. They consist of two subunits, the 50S and 30S subunits, each composed of RNA and proteins. Ribosomes facilitate the translation of mRNA into proteins by assembling amino acids according to the genetic code. They serve as the “engines” of protein synthesis in bacteria.

Question 37

What is the significance of bacterial RNA and its targeting by antibiotics?

Answer 37

Bacterial RNA is crucial for protein synthesis and is a target for many antibiotics. Antibiotics can bind to bacterial RNA and interfere with the functions of ribosomes, inhibiting protein synthesis. This disruption of bacterial RNA and protein synthesis is a common mechanism of action for antibiotics.

Question 38

What are plasmids and transposons?

Answer 38

Plasmids are circular, extra-chromosomal DNA molecules that exist independently from the bacterial chromosome. They can replicate autonomously and carry additional genes, which can include antibiotic resistance genes or genes coding for toxins. Plasmids can be passed down to progeny during cell division and can also be transmitted between bacteria.

Question 39

What are spores and which bacteria produce them?

Answer 39

Spores are a non-replicating dormant form produced by certain medically important bacteria, such as Bacillus species and Clostridium species. Spores can be thought of as “seeds” that allow bacteria to survive in unfavorable conditions. They are highly resistant to drying, temperature extremes, disinfection, and digestion.

Question 40

What is the significance of spores in clinical disease and infection control?

Answer 40

Spores play an important role in the clinical disease patterns caused by spore-forming bacteria. They can contribute to the persistence and spread of infections. Additionally, spores are highly resistant, allowing them to survive in the environment for extended periods, which presents challenges in infection control measures.

Question 41

How do bacteria regulate their genes?

Answer 41

Bacteria possess mechanisms to sense and adjust to their environment. They can regulate gene expression to adapt to changing conditions. Gene regulation in bacteria includes adjusting growth rates, regulating metabolic pathways, and controlling the production of virulence factors. Bacteria can modulate the expression of adhesion molecules, enzymes that degrade host proteins, enzymes that degrade immune mediators, and factors that can lyse host cells. The regulation of genes is critical for bacterial survival, colonization, and causing infections. Different growth phases, such as the exponential and stationary phases, can influence the expression of virulence factors and the disease outcomes.