Viruses and Cellular Dependency

• Microbiology is the study of microorganisms, which are microscopic organisms.
• The four types of microorganisms are viruses, bacteria, protists, and fungi.

Viruses

• Viruses are Latin for "poison."
• Viruses are lifeless particles that are unable to carry out metabolic functions on their own and cannot reproduce on their own.
• Viruses can reproduce once they invade a living cell.
• Viruses are measured in nanometers (nm).
• One nm equals one-billionth of a meter.
• Viruses range in size from 20 nm to 400 nm.
• Over 5000 influenza viruses can fit on the head of a pin.
• A virus consists of an inner nucleic acid strand of DNA or RNA surrounded by an outer protective protein coat called a capsid.
• The capsid accounts for 95% of a virus's total composition.
• Viruses attack normal cells and destroy them causing "symptoms".
• Viral infections are difficult to treat and antibiotics do not destroy them.
• Vaccines stimulate the immune system to fight back against disease.
• Vaccines are liquid preparations of dead or weakened viral cells, toxins, or RNA that can prevent some diseases such as polio, smallpox, and hepatitis B.
• DNA viruses cause smallpox and chickenpox.
• RNA viruses cause measles, mumps, the common cold, influenza (flu), HIV/AIDS, polio, and hepatitis through fecal contamination.
• Host range refers to the variety of cells that a virus can enter.
• Specific viruses can only enter specific cells.
• Broad host range means a virus can infect a wide range of cells/species, such as the rabies virus that infects mammals, birds, and reptiles.
• Narrow host range means a virus can only enter specific cells, such as the common cold virus which can only enter cells of the upper respiratory tract.

Bacteria

• Bacteria are microorganisms that are unicellular, though some stick together in colonies.
• Bacteria lack nuclei, vacuoles, mitochondria, and chloroplasts.
• They have a single chromosome in the form of a DNA loop and a rigid outer wall.
• Bacteria reproduce asexually without needing sexual partners.
• Bacteria thrive in moist environments, becoming inactive in a dry environment.
• Bacteria range in size from 0.4 micrometers to several micrometers (µm).
• Bacteria are prokaryotic, meaning they are single-celled, lack a nucleus, and have genetic material floating freely inside the cell.
• A plasmid is a small DNA molecule within a cell that can replicate independently.
• Bacterial cells can exchange plasmids to gain information such as antibiotic resistance.
• Prokaryotic cells can be divided into two groups: archaebacteria and eubacteria.

Archaebacteria

• Archaebacteria thrive under extreme conditions, such as without oxygen, in extremely hot and saline environments.
• Archaebacteria can be found in the intestines of mammals, volcanic deep-sea vents, and salt flats.

Eubacteria

• Eubacteria contain peptidoglycan in their cell walls.
• When stained with a purple dye and a red dye, two types can be seen: gram-negative and gram-positive.
• Gram-negative bacteria have less peptidoglycan and an extra outer layer, so they do not retain the purple dye and appear pinkish-red.
• Gram-positive bacteria have more peptidoglycan, retain the purple dye, and appear purple.
• Eubacteria appear as spheres, rods, curves or spirals and perform either anaerobic or aerobic respiration.
• Eubacteria can be autotrophs that make their own food from inorganic substances or heterotrophs who obtain their nutrients from other organisms.
• Photosynthetic autotrophs convert carbon dioxide and water into carbohydrates by using energy from sunlight.
• Chemosynthetic autotrophs use chemical reactions as their energy source.

Protists

• Protists are extremely diverse in shape, size, structure, complexity, feeding habits, locomotion, reproduction, and habitat.
• Most protists are unicellular.
• Protist cells are eukaryotic, having a membrane-bound nucleus.
• Protists mostly reproduce asexually and thrive in moist surroundings, such as fresh and salt water, animal fluids, and damp terrestrial environments.
• The three distinct groups of protists are plant-like, animal-like, and fungi-like protists.

Plant-Like Protists

• Plant-like protists have organisms that contain chlorophyll and obtain energy through photosynthesis.
• The two groups of plant-like protists are Euglena and algae.
• Euglena are typical plant-like protists with two key features: an eyespot, believed to detect light, and a flagellum, used to propel the organism through water in a whip-like fashion.
• Algae can be green, brown, and red and are normally considered aquatic organisms.
• Algae can also be located in soils, on lower trunks of trees, and on rocks.

Algae

• Brown and red algae are seaweeds

Animal-Like Protists

• Animal-like protists, also called protozoa, are all heterotrophs classified by their type of locomotion.
• The four types of animal-like protists are pseudopod, ciliated, flagellated, and parasitic protozoans.
• Pseudopod protozoans, such as amoebae, are large but simple and move by repeatedly extending and retracting their pseudopods, feeding by phagocytosis.
• Ciliated protozoans, such as paramecium, use hair-like structures for mobility and are the most advanced type.
• Flagellated protozoans, such as vorticella, move by whipping their flagellum.
• Parasitic protozoa depend entirely on the body fluids of their hosts for movement and cannot live in an open environment on their own.

Fungi-Like Protists

• Fungi-like protists, also called slime moulds, prefer cool, shady, and moist places and are usually found under fallen leaves or on rotting logs.
• The name comes from the slimy trail left behind as mould travels over the ground.
• At times, slime moulds resemble protozoans and become amoeba-like or have flagella.
• At other times, they produce spores much as fungi do.

Fungi

• Fungi were once classified as plants, but do not photosynthesize.
• Usually, the only visible parts of fungus are the reproductive structures.
• Plants have one nucleus per cell, are mostly autotrophs, and have starch as their main storage molecule, as well as having roots and cellulose in their cell walls, and some reproducing by seed.
• Fungi often have many nuclei per cell, are heterotrophs, have few or no storage molecules, have no roots, have chitin in their cell walls, and none reproduce by seed.
• Fungi have mycelium, which is a mesh of tiny branching filaments called hyphae that exist where the organism is attached to the substrate and provide transportation to pass nutrients to the fungal body.
• Hyphae can have three possible appearances: with cross-walls separating nuclei, without cross-walls to separate nuclei, and mycelium showing many interlocking hypha.
• The types of fungi are common moulds, yeast, morels, truffles, mushrooms, shelf fungi, and parasitic fungi.

Virus Reproduction

• Viruses reproduce by replication, which involves five steps to duplicate genetic material: attachment, penetration, synthesis of protein and nucleic acids, assembly of the units, and release of new virus particles.
• A virus chemically recognizes a host cell, attaches to it, and penetrates it, entering either the whole virus or only its genetic material into the cell's cytoplasm.
• Information in viral DNA or RNA directs the host cell in replicating viral components such as nucleic acids, enzymes, and capsid proteins.
• Viral nucleic acids, enzymes, and proteins are brought together and assembled into new viruses, and the newly formed virus particles are released from the infected cell.
• The host cell dies (lysis), with this entire process being known as the "lytic cycle" that can be completed in as little as 25 to 45 minutes.

Bacterial Reproduction

• Bacteria reproduce in two ways; asexually by binary fission and sexually by conjugation.

Binary Fission (Asexual)

• Binary fission bears some resemblance to mitosis but is much simpler.
• A single strand of DNA replicates so identical genetic material is transferred.
• A crosswall is produced, dividing the cell into two identical bacteria which may separate or remain attached.

Conjugation (Sexual)

• Conjugation is not as common as binary fission, but does occur in some bacteria, like E. coli and Salmonella where donor and recipient bacteria make cell-to-cell contact.
• Across a special structure called a sex pilus, genetic material is swapped giving each recipient an altered set of characteristics and then the two bacteria separate.

Protist Reproduction

• Protists reproduce in three possible ways: asexually by binary fission (like amoebae), asexually by longitudinal fission after nuclear division where the cell divides lengthwise (like euglena), and only rarely do some reproduce sexually by conjugation.

Fungal Reproduction

• Fungi reproduce both asexually and sexually, having a life cycle which is the development of an organism from a single cell to its reproductive stage.
• The fungal cycle is complete when the organism producers the next generation.
• Fungi always produce spores, which are reproductive cells in specialized reproductive structures called sporangia.
• Spores are usually dispersed by air currents.
• When spores are in a suitable environment, they sprout or germinate.

Explore the unique characteristics of viruses, including their nanometer size and obligate intracellular lifestyle. Understand why viruses are considered non-living outside a host cell. Learn to classify microorganisms based on their size and dependency on host cells for replication.