Chapter 27 | Bacteria and Archaea

Bacteria

One of two prokaryotic domains, the other being Archaea

Peptidoglycan

A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides

Gram-negative

Bacteria with walls that have less peptidoglycan and are more structurally complex, with an outer membrane that contains lipopolysaccharide (lipid connected to a long polysaccharide). Stain pink-red with gram stain

Gram-positive

Bacteria with relatively simple walls composed of a thick layer of peptidoglycan. Stain purple with gram stain

Endospore

A thick-coated, resistant cell produced by some bacterial cells when they are exposed to harsh conditions

Capsule

In many prokaryotes, a dense and well-defined layer of polysaccharide or protein that surrounds the cell wall and is sticky, protecting the cell and enabling it to adhere to substrates or other cells

Fimbriae

A short, hairlike appendage of a prokaryotic cell that helps it adhere to the substrate or to other cells

Nucleoid

A non-membrane-enclosed region in a prokaryotic cell where its chromosome is located

Plasmid

A small, circular, double-stranded DNA molecule that carries accessory genes separate from those of a bacterial chromosome

Conjugation

The direct transfer of DNA between two cells that are temporarily joined

Transformation

A change in genotype and phenotype due to the assimilation of external DNA by a cell

Transduction

A process in which viruses, known as phages, carry bacterial DNA from one bacterial cell to another

Thermophile

An organism that thrives at relatively high temperatures

Halophile

An organism that lives in a highly saline environment

Antibiotic resistance

The phenomenon where bacteria evolve to become resistant to the effects of drugs designed to kill them, often due to rapid reproduction and the spread of resistance genes within and among bacterial species, making infections harder to treat

Prokaryotes

• Evolved around 4.1 billion years ago

• Have had the most time to adapt, thriving in diverse and extreme environments

• Are the most diverse and abundant group of organisms, classified into two domains: bacteria and archaea

• Only about 2% of bacteria have been identified

Prokaryotic morphology & anatomy

• Are primarily unicellular, though some bacteria are multicellular

• They typically range from 0.5 to 5 micrometers in diameter, a small size that allows them to thrive in large numbers in limited spaces

• Their shapes include coccus (spherical), bacillus (rod-shaped), and spirillum (spiral)

Biofilms

A surface-coating colony of one or more species of unicellular organisms that engage in a metabolic cooperation

Pili

In bacteria, a structure that links one cell to another at the start of conjugation

Flagella

A long cellular appendage specialized for locomotion. Are made of a motor, hook, and filament that are synthesized by 42 proteins

Taxis

An oriented movement toward or away from a stimulus

Chemotaxis

An oriented movement toward or away from a chemical stimulus

Internal anatomy of bacteria

• Lack membrane-bound organelles

• Ribosomes responsible for protein synthesis

• A single, circular chromosome located in the nucleoid region of the cell

• Contain plasmids

Highly folded plasma membrane

These membranes include the respiratory membrane for cellular respiration and the thylakoid membrane for photosynthesis

Archaea

Known for being extremophiles

• Unique traits include membrane lipids with branched structures and the absence of peptidoglycan in their cell walls

• Archaea share some characteristics with eukaryotes, such as similarities in DNA analysis, making them more closely related to eukaryotes than to prokaryotes

Life cycle of prokaryotes

1-3 hours before undergoing division

Binary Fission

A method of asexual reproduction in single-celled organisms in which the cell grows to roughly double its size and then divides into two cells

Mutation

The exchange, deletion, or insertion of one or more nucleotides in DNA

• This process leads to genetic variation

• Mutation rates in prokaryotes are not higher than in eukaryotes; however, due to their short life cycles, mutations can be incorporated into prokaryotic populations much more quickly

Conjugation using plasmid

• Requires genes for protein production that allow material exchange, such as the F-plasmid

• The F-plasmid creates a mating bridge by forming a pilus between two bacterial cells

• The pilus extends, attaches, and retracts to bring the cells closer together

• One strand of the F-plasmid is transferred from the donor (F+ cell) to the recipient (F- cell) through the mating bridge

• The strands replicate, forming double strands, so both cells now contain the DNA from the F-plasmid

Conjugation using bacterial chromosomes

• Requires the F-factor integrated into the bacterial genome

• A pilus and mating bridge form between the two cells

• A single strand of DNA is transferred from the donor to the recipient

• Crossing over and recombination occur, incorporating the new DNA into the recipient's chromosome

Phototroph

An organism that uses light as its primary energy source

Chemotroph

An organism that obtains energy by the oxidation of electron donors in their environments

Autotroph

An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms

Heterotroph

An organism that obtains organic food molecules by eating other organisms or substances derived from them

Photoautotroph

An organism that harnesses energy to drive the synthesis of organic compounds from carbon dioxide

Chemoheterotroph

An organism that requires organic molecules for both energy and carbon

Chemoautotroph

An organism that obtains energy by oxidizing inorganic substances and needs only carbon dioxide as a carbon source

Photoheterotroph

An organism that uses light to generate ATP but must obtain carbon in organic form

Aerobic respiration

A chemical process that uses oxygen to convert glucose into energy, carbon dioxide, and water

Anaerobic respiration

A process that produces energy without oxygen

Obligate aerobe

An organism that requires oxygen for cellular respiration and cannot live without it

Obligate anaerobe

An organism that carries out only fermentation or anaerobic respiration. Such organisms cannot use oxygen and in fact may be poisoned by it

Nitrogen fixation

The conversion of atmospheric nitrogen to ammonia

Heterocyst

A specialized cell that engages in nitrogen fixation in some filamentous cyanobacteria

Metabolic cooperation

Individual prokaryotic cells within a community share resources and metabolic processes, often by exchanging metabolites, allowing them to perform specialized functions and enhancing their collective survival, most commonly observed in biofilms

Methanogen

An organism that produces methane as a waste product of the way it obtains energy

Symbiosis

An ecological relationship between organisms of two different species that live together in direct and intimate contact

Mutualism

An ecological interaction that benefits individuals of both interacting species

Commensalism

An ecological interaction that benefits the individuals of one species but neither harms nor helps the individuals of the other species

Parasitism

An ecological interaction in which one organism, the parasite, benefits by feeding upon another organism, the host, which is harmed

Pathogen

An organism or virus that causes disease

Exotoxins

A toxic protein that is secreted by a prokaryote or other pathogen and that produces specific symptoms, even if the pathogen is no longer present

Endotoxins

When the lipopolysaccharides from the outer membrane of gram-negative bacteria are released when the bacteria die

Decomposition & Recycling

The breakdown of organic matter by bacteria and other organisms, releasing nutrients back into the environment. This process recycles carbon, nitrogen, and phosphorus: carbon is released as CO2 for photosynthesis, nitrogen returns as nitrate and nitrite, and phosphorus is recycled similarly. Humans utilize bacteria in bioremediation and sewage treatment to break down waste. Additionally, bacteria help degrade biodegradable plastics, and some bacteria can absorb metals for environmental cleanup

Facultative anaerobe

An organism that makes ATP by aerobic respiration if oxygen is present but that switches to anaerobic respiration or fermentation if oxygen is not present

Fermentation

An anaerobic process in which energy can be released from glucose even if oxygen is not available

Horizontal Gene Transfer (HGT) or Lateral Gene Transfer

The transfer of genes from one genome to another through transformation, transduction, and conjugation