Microorganisms and Bacteria Overview
1.1 MICROORGANISMS
Definition of a Microorganism
An organism too small to be seen without the aid of a microscope.
Relatively simple in structure and often unicellular (single-celled).
Also called a "microbe," "germ," or "bug." In the medical community, "bug" refers to an infection-causing microorganism, not an insect.
Four Groups of Microorganisms
Protozoa
Fungi
Bacteria
Viruses
1. Protozoa
Characteristics
Unicellular creatures.
Move either by flagella or amoeboid motion.
Each cell has a nucleus and is enclosed by a plasma membrane.
Live in water and soil, feeding on bacteria and small particles.
Disease-Causing Protozoa
Some protozoa live in our bodies harmlessly, but a few can cause disease.
Giardia infections: commonly known as "beaver fever."
Giardia attach to the intestinal wall using two sucking discs.
This results in diarrhea because food cannot be properly broken down and absorbed.
Giardia move by flagella (as shown in Figure 1.1.1).
Amoeba: Some amoeba invade the intestinal wall, causing diarrhea. Occasionally, they move into deeper tissue.
These are usually acquired in warm countries with poor sanitation.
Amoeba move by the extension of pseudopods and do not have flagella.
2. Fungi
Characteristics
Can be considered non-photosynthetic plants.
Each cell has a nucleus and is enclosed by a rigid cell wall.
A very diverse group of microorganisms.
Divisions of Fungi
Yeasts
Unicellular oval structures.
Reproduce by budding.
Many types are used in the food and beverage industry for making breads and wines.
All yeast types appear similar under a microscope.
Disease-causing yeasts: Some yeasts can cause disease in humans, with Candida being the most common.
Candida can cause oral thrush, vaginal discharge, skin infections, pneumonia, and even death.
Molds
Multicellular structures that form visible clumps of growth.
Examples include bread and cheese molds.
Molds begin as long tubular structures that eventually produce spores.
These spores are visible as grey, blue-green, or black growth.
Human infections: Mold-type fungi typically cause skin infections, such as Athlete's foot and ringworm.
3. Bacteria
Characteristics
Tiny unicellular organisms.
Typically surrounded by a rigid cell wall.
Do not have an organized nucleus but perform all necessary activities for growth and reproduction.
Ubiquitous: Found almost everywhere there is moisture and nutrients.
Many bacteria are part of our natural body flora and are beneficial.
Disease-Causing Bacteria
Streptococcus: causes sore throat (often called "Strept throat").
Staphylococcus: certain types cause skin abscesses called boils.
Salmonella: commonly associated with food poisoning from poultry.
Figure 1.1.5 illustrates the microscopic appearance of bacteria.
4. Viruses
Characteristics
Even smaller than bacteria and have a very simple structure.
Unable to grow and reproduce on their own; they must rely on a living host cell to replicate viral parts.
Animal, plant, and bacterial cells can all serve as host cells for viruses.
Disease-Causing Viruses
Influenza and colds: common viral infections causing discomfort.
More serious viral infections: hepatitis, rabies, and HIV.
Treatment of Microorganism Infections
Each group of microorganism requires a different drug for treatment when causing an infection.
Protozoal infections: treated with antiprotozoal drugs.
Fungal infections: treated with antifungal drugs.
Bacterial infections: treated with antibiotics.
Viral infections: treated with antiviral drugs.
Size of Microorganisms
Units of Measurement
Most microorganisms are measured in micrometers (often denoted as \mu m).
One micrometer equals 1/1,000 of a millimeter (1\text{ mm} = 1,000\text{ }\mu m).
Examples of Size and Quantity
Staphylococci bacteria on the skin are about 1 micrometer in diameter.
Approximately 1,000 Staphylococci would fit in a row on 1 millimeter of a ruler.
Human skin is covered with these bacteria; there are an estimated 16 trillion in the mouth alone, mostly beneficial.
General Size Range
All microorganisms are smaller than 0.1 mm, which equals 100 micrometers.
However, there is considerable size variation (as illustrated in Figure 1.1.7).
Typical Sizes by Group
Protozoa: 15 - 20 micrometers
Fungi: 5 - 10 micrometers
Bacteria: 0.3 - 5 micrometers
Viruses: 0.02 - 0.2 micrometers
1.2 BACTERIA
Most disease-causing microorganisms found in hospital settings are bacteria.
Shapes of Bacteria
Bacteria can be categorized into three groups based on their shape, which is maintained by their rigid cell wall.
Cocci (plural), coccus (singular):
Spherical or round cells.
Rods or bacilli (bacillus):
Rectangular-shaped cells.
Spirilla (spirillum):
Curved or spiral-shaped rods.
Gram Reaction of Bacteria (Gram Stain)
Gram Stain Procedure
A staining technique used for over 100 years to make bacteria more visible under a microscope.
Divides bacteria into two groups based on their reaction to the stain.
Results
Gram-positive: appear dark bluish-black in color.
Gram-negative: appear pink to red in color.
Importance of the Gram Reaction
Identification: Looking at the gram reaction and shape is often the first step in identifying bacteria in the laboratory.
Bacteria can be divided into six groups:
Gram-positive cocci
Gram-negative cocci
Gram-positive rods (bacilli)
Gram-negative rods (bacilli)
Gram-positive spirilla
Gram-negative spirilla
Antibiotic Effectiveness: The gram reaction determines the effectiveness of certain antibiotics.
For example, Penicillin G is effective against gram-positive bacteria but relatively ineffective against gram-negative bacteria.
Gram stain results assist physicians in the initial selection of an appropriate antibiotic.
Disinfectant Effectiveness: The gram reaction also determines the effectiveness of certain disinfectants, with some being more effective against gram-positive than gram-negative bacteria.
Bacterial Endospores
Formation
A small number of bacteria (primarily a few Gram-positive rods) can produce a special type of spore within the bacterial cell.
These are called endospores to distinguish them from fungal spores (borne on the ends of hyphae).
Bacterial cells without endospores are called vegetative cells and are actively growing and multiplying.
Sporulation (endospore formation) occurs when certain nutrients are depleted.
Structure
One copy of the genetic material and a tiny amount of cytoplasm are enclosed by an insulating layer.
The entire structure is covered with several compact layers of spore coat.
Survival and Germination
Endospores can remain dormant for days, months, or even years without nutrients or moisture.
Many bacteria found in dust, cereals, grains, and soil exist as endospores.
A "viable" endospore can germinate (grow) into a vegetative cell when moisture and nutrients become available.
One spore germinates into one vegetative bacterium.
Diseases Caused by Spore-Forming Organisms
Spores are ubiquitous (found everywhere) in the soil.
They normally do not cause infections as they require special conditions to vegetate, such as a lack of oxygen.
Gangrene and Tetanus: Result when spores are introduced deep into injured tissue where blood flow is disrupted, and oxygen levels are low.
Anthrax: Will grow in the presence of oxygen, but requires other pre-disposing conditions like damaged tissue or inhalation into the lungs.
Significance of Endospores in Sterilization and Disinfection
Heat Resistance: Endospores are highly resistant to heat.
Most vegetative bacteria are killed by moist heat at 60-80^ ext{\circ} C for 10 minutes, but spores survive these temperatures.
Some spores are killed by boiling, but others require a temperature of 121^ ext{\circ} C for 12-15 minutes for destruction.
Disinfectant Resistance: Endospores are more resistant to disinfectants than vegetative bacteria.
Low-level disinfectants may not kill endospores, and high-level disinfectants require extended exposure times.
Cold Resistance: Endospores are very resistant to destruction by cold (viable spores have been found in mammoths from Siberian glaciers).
Other Resistances: Endospores are also resistant to ultraviolet light, acids, alkalis, and detergents.
GROWTH OF BACTERIA
Binary Fission
Definition: The primary method of bacterial reproduction where one bacterium divides into two identical daughter cells.
Generation Time
The time it takes for binary fission to occur, meaning the time it takes for a bacterial population to double.
Examples: One bacterium becomes two, forty bacteria become eighty.
Variability: Generation time is not constant for all bacteria and is affected by temperature and available nutrients.
Rapidly growing bacteria under ideal conditions have a generation time of 15-30 minutes.
Relation to Disease Rate
Bacteria causing gas gangrene have a very short generation time (about 8 minutes), leading to rapid tissue destruction (e.g., a limb destroyed in a day).
Bacteria causing tuberculosis have a long generation time (12-24 hours), requiring weeks to produce disease.
Relation to Visible Growth
Generation time also determines the time needed for bacteria to form visible growth on culture media.
A colony is a visible mass of bacteria that develops on the surface of a solid culture medium, representing all descendants of a single bacterial cell.
Rapidly growing bacteria form colonies within 18-24 hours.
The culture medium provides necessary nutrients for bacterial growth.
Bacterial Growth Curve
Bacteria do not grow at their maximum rate indefinitely due to limited nutrients, space, and the buildup of toxic waste products.
If one cell divided every 20 minutes for 24 hours, there would be 1 \times 10^{21} cells with a mass of 4,000 tons.
The maximum number of bacteria possible is about 1 \times 10^9 per mL or gram.
Phases of Growth (when bacteria are introduced into a new environment)
Lag Phase
Little or no increase in numbers during this short period.
Cells adapt to the new environment.
Typically a few hours, but can vary.
For bacteria in food, the lag phase is about 2 hours, allowing food to be left out for this duration without significant worry of bacterial growth and food poisoning.
Log Phase (Exponential Phase)
A period of maximum bacterial growth.
All cells divide at a constant, rapid rate.
Stationary Phase
Bacteria exhaust their nutrient supply and stop growing and multiplying.
Accumulation of toxic waste products can also prevent further growth.
The number of live bacteria remains constant during this phase.
Death Phase (Phase of Decline)
Bacterial cells begin to die, and the number of live cells decreases.
Most bacteria may die quickly, but some can linger for weeks, months, or even years.
Spore-forming bacteria are the ones that can survive the longest.
The Growth Curve Related to Infection
The same sequence of events observed in laboratory cultures often occurs when a microorganism invades a host.
Incubation Period (corresponds to the Lag Phase)
Time immediately following microorganism introduction into the host with no disease symptoms.
Acute Stage (corresponds to the Log Phase)
Onset of symptoms (e.g., fever, sore throat, swollen lymph glands, rash, vomiting, diarrhea) depending on the microbe.
Most patients recover with the help of host defenses and medical intervention.
If these fail, the patient may die during this stage.
Stationary Phase (of disease)
A period where symptoms are neither worsening nor improving.
Death Phase (of disease) (corresponds to the Death Phase of microorganism)
Symptoms subside, and recovery begins (death of the microorganism, not the host).
Convalescent Period
Covers the time needed for complete recovery, which varies with different diseases.
Infectious microorganisms may continue to be discharged during this period.
1.3 VIRUSES
Viral Characteristics
Viruses are fundamentally different from bacteria, with two distinguishing characteristics:
Obligate Intracellular Parasites
They only multiply when inside a living host cell.
Cannot reproduce outside a host cell.
Do not multiply in non-cellular environments like foods, water, bacterial culture media, or medications.
Single Type of Nucleic Acid
Contain either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
All other forms of life contain both types of nucleic acid.
Structure
Possess a protein coat (capsid) surrounding the nucleic acid.
Some viruses also have an outer envelope composed mainly of lipids.
Others may have spikes protruding from the lipid envelope.
Lipid Viruses
Generally easier to inactivate with disinfectants.
Exception: Hepatitis B is an enveloped virus that is quite resistant to destruction and can remain infective in dried body fluids for 7 days.
Multiplication of Animal Viruses
For a virus to multiply, a specific sequence of events must occur in the host cell:
Attachment
The virus must attach to complementary receptor sites on the surface of the host's cells.
Example: Adenoviruses (colds) attach to epithelial cells of the respiratory tract. The AIDS virus attaches to receptor sites on specific white blood cells called lymphocytes.
Entry
After attachment, the virus is taken into the host cell by endocytosis.
The cell's membrane folds inward, forming a vesicle containing the virus.
Uncoating
The host cell attempts to destroy the vesicle's contents, digesting the outer protein coat.
The viral nucleic acid is then released inside the host cell.
Replication and Synthesis
The viral nucleic acid carries the genetic coding for new viral material and directs the synthesis of viral parts.
The virus utilizes the enzymes and metabolic pathways of the host cell for replication.
Most DNA viruses replicate DNA in the cell's nucleus and proteins in the cytoplasm.
Proteins then move to the nucleus and join with the DNA.
Assembly and Release
New viral particles are assembled.
Lipids and other viral components may be added as the virus particle is released from the host cell.
Influenza Viruses
Influenza A viruses
Responsible for pandemic flu outbreaks, which spread rapidly worldwide and cause many deaths.
Dangerous due to quick and frequent mutations.
Individuals lack immunity, making timely vaccine production difficult.
Examples: 1918 Spanish flu (H1N1), Asian flu (H2N2) of 1957-58, Hong Kong flu (H3N2) of 1968-69, H1N1 swine flu of 2009.
Influenza B and C viruses
Cause local outbreaks.
Generally less severe with a lower mortality rate than Influenza A outbreaks.
Common Cold
Not caused by flu viruses.
Majority are caused by rhinoviruses.
Symptoms: sneezing, runny nose, nasal congestion, scratchy or sore throat, coughing, headache, and fatigue.
Distinction between Cold and Flu Symptoms
Influenza symptoms are more severe, including rapidly rising fever, chills, and body/muscle aches.
Effects of Viruses on Host Cells
Host Cell Death: Infection typically kills the host cell through:
Lysis: The cell bursts when it fills with viral particles.
Metabolic Diversion: The cell's metabolic pathways are redirected to manufacture viral particles instead of normal cellular functions.
Immune System Destruction: The virus-infected cell is destroyed by the lymphocytes of the cellular immune system. This can be detrimental if the infected cells are vital for survival (e.g., liver cells infected with Hepatitis B virus).
Transformation into Tumor Cells: A few viruses can alter the host cell's nucleic acid, transforming it into a tumor cell.
These are called oncogenic viruses.
Hepatitis B is considered an oncogenic virus due to its association with liver cancer.
Control of Viruses
Antibiotics: Have no effect on viral replication because they are directed against bacterial metabolic pathways. Viruses lack their own metabolic activities.
Antiviral Drugs: While many drugs that inhibit viral replication would also destroy host cells, a few useful antiviral drugs exist.
Acyclovir: Well-known for controlling genital herpes.
Zidovudine (ZDV) (formerly Azidothymidine or AZT): Useful in controlling the replication rate of the AIDS virus; approved for HIV treatment in the late 1980s.
Paxlovid: An antiviral used to treat mild to moderate COVID-19 in adults at high risk for hospitalization/death.
Research into antiviral agents is highly active, including for AIDS and COVID-19$$ treatment.
Immunization: Effective for many viral diseases.