Introduction to Microbiology

Introduction to Microbiology

• Microbiology is a specialized area of biology studying living things usually too small to see without aid

Microorganisms

• Microorganisms include bacteria, fungi, viruses, algae, protozoa, and helminths

Importance of Microbes

• Microbes are ubiquitous and found everywhere, from soil to extreme environments and within other organisms
• They function as photosynthesizers, decomposers, and recyclers

Terminology

• Pathogens are microorganisms capable of causing disease in a host
• Opportunistic pathogens usually do not harm the host but cause disease when the host's defenses are compromised like Pseudomonas aeruginosa
• Virulence is a pathogen's relative capacity to invade and harm host cells
• Virulence factors are bacterial features that help infect a host, avoid the immune system, and assist in colonization, such as adhesins, toxins, and enzymes
• Spreading factors are extracellular enzymes and toxins that destroy host tissue and allow bacterial spread, like hyaluronidase and collagenase
• Commensals/Normal flora are microbes colonizing a host without causing damage and are generally harmless and do not invade tissue/blood

Location and Normal Flora

• Blood and tissue typically have no normal flora
• Skin: Staphylococcus epidermidis
• Oropharynx: Viridans streptococci
• Gastrointestinal tract: Escherichia coli, Bacteroides species
• Vagina: Lactobacillus species
• Urethra: Staphylococcus epidermidis

History of Microbiology

• Antonie Van Leeuwenhoek was the first to describe bacteria in 1976, discovered "animalcules", built the microscope, and is considered the Father of Microbiology
• Louis Pasteur developed the Theory of Biogenesis vs. Spontaneous Generation and invented Pasteurization
• Robert Koch demonstrated the link between microbes and infectious diseases, identified causative agents of anthrax and tuberculosis, and developed techniques for obtaining pure cultures
• Koch's postulates are four steps to confirm if a suspected pathogen is the cause of a disease
• The pathogen must be present in all cases of the disease and absent from healthy animals
• The pathogen must be isolated from the diseased host and grown in pure culture
• The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal; the disease must be reproduced when a pure culture of the organism is introduced into a healthy, susceptible host
• The pathogen must be isolated from the inoculated animal and must be shown to be the original organism; the same organism must be reisolated from the experimentally infected host
• Sir Alexander Fleming discovered penicillin

Microbial Classification

• Taxonomy is organizing, classifying, and naming living things
• Concerned with organizing organisms into groups at the levels of Domain, Kingdom, Phylum/Division, Class, Order, Family, Genus, and Species
• Nomenclature involves assigning names using a binomial (two-name) system, e.g., Staphylococcus aureus
• Identification involves determining and recording traits of organisms for taxonomic placement

Prokaryotes vs. Eukaryotes

• Prokaryotes lack a membrane-bound nucleus, and their DNA is free in the cytoplasm within a nucleoid region
• Eukaryotes have a true membrane-bound nucleus with DNA bound by a nuclear membrane and a nucleolus
• Prokaryotes have circular, double-stranded DNA with only exons
• Eukaryotes have linear, double-stranded DNA with exons and introns
• Prokaryotes lack histones in DNA and do not have organized chromosomes
• Eukaryotes have DNA complexed with histones and organized into more than one chromosome
• Prokaryotes may contain plasmids, while plasmids are found only in yeasts in eukaryotes
• Prokaryotes divide asexually by binary fission, whereas eukaryotes divide by mitosis
• Prokaryotes reproduce sexually via transfer of genetic material by conjugation, while eukaryotes reproduce sexually by meiosis
• Prokaryotes possess a cytoplasmic membrane without carbohydrates or sterols
• Eukaryotes have a cytoplasmic membrane with sterols and carbohydrates
• Prokaryotes lack membrane-bound organelles; respiration and electron transport occur in the cytoplasmic membrane
• Eukaryotes have membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus
• Prokaryotic flagella consist of two protein building blocks, whereas eukaryotic flagella are complex with multiple microtubules
• Prokaryotic ribosomes are 70S (smaller), while eukaryotic ribosomes are 80S (larger), except for the 70S ribosomes in mitochondria and chloroplasts
• Prokaryotes are unicellular, while eukaryotes are often multicellular

Microscopy

• The objective lens (closest to the specimen) and ocular lens/eyepiece function together to resolve the image
• The initial image is a real image, and the second is a virtual image that produces the final magnified image
• Total power of magnification is the power of the objective lens times the power of the ocular lens
• A bright-field microscope light path consists of a transillumination light source, a condenser, and ocular lenses
• Maximum magnification is 1000x
• It allows observation of cell morphology (shape/arrangement) and intracellular structures
• The main advantage is that living specimens can be examined
• A disadvantage is that staining kills the microbes

Unicellular Prokaryotic Organisms : Morphology

• Shape is determined by a rigid wall
• Spherical/round cells are coccus (plural: cocci)
• Cylindrical/rod-shaped cells are bacillus (plural: bacilli) or rods
• Helical shapes include twisted/spiral spirochaetes and spirillum

Arrangement

• Arrangement (style of grouping) depends on the plane of successive cell division
• General arrangements include chains, clusters, and palisades (angled pairs)
• Single coccus
• Pairs (diplococci) are cocci that remain in joined pairs after dividing
• Chains (streptococcus) are cocci that divide along one plane and remain connected
• Packets/tetrads are cocci that divide in two planes and remain in groups of four
• Sarcina are cocci that divide in three planes and remain in cube-like groups of eight
• Clusters (staphylococcus) are cocci that divide in multiple planes and form grape-like clusters

Bacillus

• Single (bacillus) where most bacilli appear as a single rod
• Chains of bacillus appear in chains after division
• Comma-shape (vibrio)
• Palisades are bacteria cells at weird angles, resulting from snapping apart their outer cell wall
• Short rods (coccobacilli) are bacilli that are so short and fat they look like cocci

Helical Shapes

• Spirillum (plural:Spirilla) have a helical shape and fairly rigid bodies
• Spirochaetes have a helical shape and flexible bodies that are long, tender, and tightly coiled like telephone wires

Pleomorphic Bacteria

• Pleomorphic bacteria do not have any characteristic shape
• Observed to have different shapes in pure cultures
• Mycoplasma pneumoniae is an example