histo lesson 2

Growth Requirements of Bacteria

  • The growth of bacteria is influenced by several key requirements, including:

    • Atmosphere

    • Temperature

    • pH

    • Nutrients

    • Incubation time

    • Note: Nutrients and incubation time will be discussed in another lesson.

Atmospheric Requirements of Bacteria

  • Bacteria have varying requirements for oxygen (O2) and/or carbon dioxide (CO2). They can be classified based on their oxygen needs and tolerances:

    • Aerobes

    • Obligate Aerobes

    • Facultative Anaerobes

    • Aerotolerant Anaerobes

    • Obligate Anaerobes

    • Microaerophilic Bacteria

Aerobes

  • Microorganisms capable of growing in an oxygenated environment.

  • Utilize gaseous oxygen in their metabolism and have the necessary enzymes to manage toxic byproducts of oxygen.

  • Examples include:

    • Escherichia coli

    • Pseudomonas aeruginosa

    • Mycobacterium tuberculosis

  • The atmospheric composition of Earth is approximately 21% oxygen.

Obligate Aerobes

  • Completely reliant on atmospheric oxygen for growth.

  • Utilize cellular respiration to oxidize substances like sugars or fats to generate energy.

    • Oxygen acts as the terminal electron acceptor in this process.

    • The pyruvate resulting from glycolysis is further metabolized via aerobic respiration.

    • Stages of Aerobic Respiration include:

    1. Glycolysis: Breakdown of glucose via enzymes, producing energy and pyruvic acid.

    2. Krebs Cycle: Citric acid cycle involved in aerobic respiration.

    3. Electron Transport Chain: Yields ATP (adenosine triphosphate), the primary energy currency of the cell.

Facultative Anaerobes

  • Organisms that do not require oxygen for growth but can utilize it effectively and grow well in its presence.

  • Capable of ATP production by either aerobic respiration in the presence of oxygen or fermentation in its absence.

What is Fermentation?

  • A metabolic process that enables respiration in the absence of oxygen, allowing cells to derive energy from glucose anaerobically.

  • Forms include:

    • Alcoholic Fermentation: Conversion of carbohydrates into ethanol and CO2 by yeast.

  • Fermentation occurs in a variety of organisms such as muscle cells, yeasts, some bacteria, and plants.

  • Examples of Facultative Anaerobic Bacteria include:

    • Staphylococcus aureus

    • Escherichia coli

Aerotolerant Microorganisms

  • Do not require oxygen for growth but can tolerate its presence.

  • Examples include:

    • Enterococcus faecalis

    • Streptococcus pyogenes

Obligate Anaerobes

  • Must avoid oxygen as it is toxic and lethal to them.

  • They produce ATP via anaerobic respiration.

Anaerobic Respiration

  • A method of generating usable energy (ATP) without utilizing oxygen.

  • Common in prokaryotes, such as bacteria, that thrive in oxygen-free environments.

  • Despite the term 'respiration,' the first step is still glycolysis. In anaerobes, other compounds such as sulfate (SO4), nitrate (NO3), and sulfur (S) may act as terminal electron acceptors rather than oxygen.

  • Obligate anaerobes are limited to respiration using these anaerobic compounds and do not survive in oxygen environments.

Identification of Anaerobic Conditions

  • Thioglycolate Broth: A medium used to demonstrate the different atmospheric requirements for bacteria.

Bacterial Oxygen Preferences

  • Obligate Aerobes: Require oxygen.

  • Facultative Anaerobes: Can grow with or without oxygen.

  • Aerotolerant Anaerobes: Can tolerate oxygen but do not use it for growth.

  • Obligate Anaerobes: Do not require and are harmed by oxygen.

Implications of Anaerobic Bacteria

  • Obligate anaerobes are a significant part of the normal microflora found in mucous membranes, such as the mouth, lower gastrointestinal (GI) tract, and vagina.

  • They can cause disease when normal mucosal barriers are compromised.

  • Anaerobic infections often result in suppurative conditions, leading to abscess formation and tissue necrosis.

  • Common tests for anaerobic conditions include:

    • Collection of specimens for anaerobic culture via aspiration or biopsy from sterile sites.

    • Quick transport to the lab using devices that prevent exposure to oxygen (using a mix of CO2, hydrogen, and nitrogen).

    • Specimens should be kept in anaerobically sterilized semi-solid media.

Clues to Anaerobic Infection

  • Gas presence in pus or infected tissues.

  • Unpleasant odor associated with pus or infected tissues.

  • Necrotic infected tissue.

  • Infection site typically located near mucosal areas where anaerobic flora thrive.

Examples of Anaerobic Bacteria

  • Bacteroides: Most common; related to intra-abdominal infections.

  • Fusobacterium: Associated with abscesses, wound infections, and infections in lungs and brain.

  • Actinomyces: Linked to head & neck infections, abdominal issues, pelvic infections, and aspiration pneumonia.

  • Clostridium (Clostridioides)

    • C. perfringens: Causes gas gangrene.

    • C. botulinum: Leads to botulism (food poisoning).

    • C. tetani: Causes tetanus.

    • C. difficile: Associated with diarrheal illnesses and colitis.

Microaerophiles

  • Microorganisms requiring oxygen at very low levels (2-10% concentration).

  • Damaged by standard atmospheric oxygen levels (21%).

  • Need a microaerophilic environment composed of approximately 80% nitrogen (N2), 10% carbon dioxide (CO2), 5% hydrogen (H2), and 5% oxygen (O2).

Examples of Microaerophiles

  • Borrelia burgdorferi: Causative agent of Lyme disease.

  • Helicobacter pylori: Responsible for peptic ulcers and specific gastritis.

  • Campylobacter jejuni: Common cause of food poisoning.

  • Streptococcus intermedius: Identified as a microaerophilic organism.

Capnophiles

  • Microorganisms that thrive in or require high concentrations of carbon dioxide for survival.

  • Examples include:

    • Campylobacter jejuni

    • Neisseria gonorrhoeae

    • Neisseria meningitidis

Laboratory Atmospheric Conditions

  • To create a defined atmospheric environment in the lab, an enclosed space is needed that can maintain specific combinations of gases.

  • Solutions for this can range from flexible pouches to rigid jars and complete incubators.

Utilization of Jars and Pouches

  • Single-use commercial gas sachets can generate controlled atmospheres in sealed jars or pouches:

    • Create anaerobic (0% O2) or microaerophilic (5-6% O2) environments.

    • Activation may involve adding water or exposure to air, depending on the product.

    • Oxygen is removed through hydrogen reacting with O2 using a palladium catalyst or chemical oxygen absorption.

    • Current systems are more reliable and faster than older methods, which often required reusable catalysts.

Anaerobic Gas Packs

  • Designed to remove oxygen, replacing it with hydrogen and CO2 inside sealed jars:

    1. Water activates the chemical components in the gas pack.

    2. The pack releases hydrogen (H2) and carbon dioxide (CO2).

    3. Hydrogen reacts with oxygen in the jar.

    4. A palladium catalyst converts H2 + O2 → H2O.

    5. This process eliminates oxygen, creating an anaerobic environment.

Modern Gas Generation Techniques

  • Updated gas packs often utilize ascorbic acid, which reacts exothermically with oxygen in the air, avoiding the need for water or hydrogen gas generation.

  • Gas jars can be filled from gas cylinders via portable gas mixing generators to reach specific atmospheric conditions, avoiding delays in target gas concentration.

Workstations for Larger Anaerobic Testing Laboratories

  • Specialized anaerobic and microaerobic workstations/chambers/incubators facilitate sample manipulation and incubation under desired environmental conditions.

  • Gases within these setups are mixed according to user-defined combinations and consistently maintained through incubation and protocol procedures.

Temperature Requirements of Bacteria

  • Bacteria have different temperature preferences:

    • Psychrophiles: Thrive at low temperatures.

    • Mesophiles: Prefer moderate temperatures.

    • Thermophiles: Favor high temperatures.

Psychrophiles

  • Extremophilic organisms growing at cold temperatures, typically from -20°C to +10°C.

  • Optimal growth temperature: 0°C to +10°C.

  • Maximum temperature tolerance: Can grow up to 20°C.

  • Minimum temperature measurement: Difficult to ascertain due to freezing of laboratory media during experimentation.

Mesophiles

  • Microorganisms thriving best in moderate temperatures, typically in the range of 25°C to 40°C.

  • Many human pathogens fall under mesophiles, with optimal growth temperatures close to normal human body temperature.

Thermophiles

  • Classified further into obligate and facultative thermophiles:

    • Obligate Thermophiles: Depend on high temperatures for growth.

    • Facultative Thermophiles: Can grow at high temperatures but also tolerate lower temperatures (below 50°C).

    • Hyperthermophiles: Extreme thermophiles whose optimal temperatures exceed 80°C, often found near underwater volcanoes at temperatures above 200°C.

Growth Rates Across Different Temperatures

  • Performance of bacteria at varied temperature levels can be depicted graphically, showcasing optimal temperature ranges for different types.

pH Requirements of Bacteria

  • Bacteria's acidic or basic growth preferences include:

    • Acidophiles: Prefer low pH (<5.5)

    • Example: Helicobacter pylori (acid-tolerant).

    • Neutrophiles: Optimally grow at neutral pH (6.5–7.5).

    • Most pathogenic bacteria fall into this category.

    • Alkaliphiles: Thrive in high pH environments (>8.5).

    • Example: Vibrio cholerae.