Summary
Highlights
Bacteria require constant nutrient intake for survival. All organisms, including bacteria, need similar basic elements like carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, sodium, chloride, and magnesium. An essential nutrient is one an organism cannot produce itself and must obtain from its environment. Nutrients are categorized into macronutrients (large quantities, e.g., carbon, hydrogen, oxygen for cell structure) and micronutrients or trace elements (small amounts, e.g., manganese, zinc for enzyme function and protein structure).
Organic compounds contain both carbon and hydrogen (e.g., glucose), while inorganic compounds lack one or both (e.g., carbon dioxide). Organic nutrients typically originate from living things or their byproducts, such as methane from bacterial digestion. The chemical makeup of a cell's cytoplasm is primarily water (70%), with proteins being the next most common component. CHOPS (carbon, hydrogen, oxygen, phosphorus, sulfur) constitute 96% of a cell's dry weight, mostly in molecular form.
Microbes are classified by their carbon and energy sources. Heterotrophs (hetero means different) obtain organic carbon from other organisms, while autotrophs (auto means self) convert inorganic carbon dioxide into organic compounds. Energy sources categorize microbes as phototrophs (light for energy) or chemotrophs (chemical compounds for energy). This leads to classifications like photoautotrophs, chemoautotrophs (chemoorganic or lithoautotrophs), photoheterotrophs, and chemoheterotrophs. Most disease-causing microbes in humans are chemoheterotrophs, deriving both carbon and energy from organic compounds through respiration or fermentation.
Chemoheterotrophs are further divided into saprobes (feed on dead organisms, aiding decomposition and nutrient recycling) and parasites (obtain nutrients from living hosts). Parasites can be ectoparasites (live on the body) or endoparasites (live within the body). All parasites are pathogens, causing harm or death to their hosts. Obligate parasites require a host to survive, unlike facultative parasites. Sodium, calcium, magnesium, iron, and zinc are other crucial elements used for processes like cell transport, stabilization, enzyme function, and more.
Molecules move due to constant Brownian motion. Diffusion is the movement of molecules from high to low concentration. This process continues until equilibrium is reached, though molecular movement never stops. Temperature affects the rate of diffusion. Osmosis is the diffusion of water across a selectively permeable membrane. Water moves to an area with a higher solute concentration. Isotonic solutions have equal concentrations inside and outside the cell, resulting in no net water movement. Hypotonic solutions have lower solute concentration outside the cell, causing water to rush in and potentially burst cells without cell walls. Hypertonic solutions have higher solute concentration outside the cell, causing water to leave the cell and shrink it (plasmolysis). Active transport requires energy to move solutes against their concentration gradient, often using specific membrane proteins like pumps. Endocytosis (phagocytosis for solids, pinocytosis for liquids) is a form of active transport where the cell engulfs substances. Facilitated diffusion uses carrier proteins but doesn't require energy as it follows the concentration gradient.
Microbes cannot regulate their internal temperature. Each species has a cardinal temperature range for growth, including a minimum, maximum, and optimum temperature. Psychrophiles (cold-loving) thrive below 15°C, growing in environments like lakes, snow, and ice, and are rarely pathogenic to humans. Psychrotrophs can grow in cold but have an optimum between 15-30°C, often causing food spoilage (e.g., *Staphylococcus aureus*). Mesophiles (middle-loving) grow best between 20-40°C, including most medically significant pathogens. Thermoduric species can survive short exposures to high temperatures but prefer mesophilic ranges. Thermophiles (heat-loving) grow optimally above 45°C, found in volcanic soils and compost. Extreme thermophiles can survive and thrive above 80°C, even in boiling water, like those found in deep-sea vents.
Oxygen has a significant impact on microbial respiration. Oxygen can be toxic due to its oxidizing properties, forming reactive byproducts like singlet oxygen, superoxide ion, hydrogen peroxide, and hydroxyl radicals. Microbes that tolerate oxygen have enzymes like superoxide dismutase and catalase to neutralize these toxic forms. Obligate aerobes require oxygen and possess detoxifying enzymes, growing at the top of test tubes. Microaerophiles need low oxygen levels. Facultative anaerobes can grow with or without oxygen, showing growth throughout the test tube. Obligate anaerobes cannot tolerate oxygen and lack detoxifying enzymes, growing at the bottom. Aerotolerant anaerobes tolerate oxygen but do not use it for metabolism, growing evenly throughout the tube. Capnophiles are species that grow best at higher-than-atmospheric levels of carbon dioxide.
The pH scale measures acidity or alkalinity. Most organisms grow between pH 6-8. Acidophiles thrive in acidic environments (e.g., *Euglena mutabilis* in acid pools). Alkalophiles prefer alkaline conditions (e.g., *Natronomonas* in hot springs at pH 12). Osmotic pressure refers to the effect of solute concentration. Osmophiles live in high solute concentrations (hypertonic solutions). Halophiles (salt-loving) prefer high salt levels, with obligate halophiles requiring at least 9% NaCl. Facultative halophiles are salt-tolerant but do not require it. Radiation (e.g., UV, ionizing) can damage microbes and is used for microbial control in industries. Barophiles (pressure-loving) live under extreme pressures, such as deep-sea microbes, and can explode at normal atmospheric pressure.
Microbes rarely live in isolation; they form complex associations. Symbiotic relationships include mutualism (both benefit), commensalism (one benefits, other is unaffected), and parasitism (one benefits, other is harmed). Non-symbiotic relationships involve free-living organisms. Synergism occurs when community members cooperate and share nutrients, producing results not achievable by single species (e.g., gum disease). Antagonism is when one member inhibits or destroys another (e.g., penicillin production by mold). Biofilms are mixed communities of microbes attached to a surface, secreting polymeric glycocalyx and using quorum sensing to coordinate behavior, making them resistant to removal and antimicrobial agents. Bacteria in biofilms exhibit different gene expression and behavior compared to free-living (planktonic) forms.
Microorganisms reproduce through binary fission, where one cell divides into two, leading to exponential growth. Growth depends on environmental conditions: temperature, moisture, nutrients, pH, oxygen, and chemical inhibitors. Under ideal conditions, bacteria can multiply rapidly; *E. coli* can double in 30 minutes. Generation time (doubling time) is the time for one fission cycle. Most pathogens have short generation times for quick infection. Bacterial growth in a closed system (finite nutrients, space, and waste removal issues) follows a predictable curve with four phases: lag phase (initial adjustment, sluggish growth), exponential/log phase (rapid, constant doubling), stationary phase (birth rate equals death rate, due to nutrient depletion and waste accumulation), and death phase (cells die exponentially). Microbes in the exponential phase are more vulnerable to antimicrobials and heat. Techniques to measure bacterial populations include turbidity (cloudiness), direct cell count using a microscope and grided slide, Coulter counter, flow cytometer (differentiates live/dead cells), and genetic probing (real-time PCR).