Bio51 Bacteria pt2

Main Ideas and Concepts

1. How Prokaryotes Obtain Energy and Carbon

Prokaryotes can be classified along two separate dimensions:

• Energy source
  • Phototrophs use light.
  • Chemotrophs obtain energy from chemicals, which may be organic or inorganic.
• Carbon source
  • Autotrophs use carbon dioxide (CO₂) as their carbon source.
  • Heterotrophs obtain carbon from organic compounds, such as sugars.

Combining these dimensions produces four nutritional modes:

• Photoautotrophs use light for energy and CO₂ for carbon. Plants, algae, and cyanobacteria are examples.
• Chemoautotrophs obtain energy by oxidizing inorganic chemicals and use CO₂ for carbon. Examples include nitrifying bacteria that use ammonia, as well as bacteria that use hydrogen sulfide or iron compounds. The lecture notes that this mode is unique to prokaryotes.
• Photoheterotrophs use light to make ATP but require organic compounds as their carbon source. The lecture describes this as a mode found in some bacteria, but not in eukaryotes.
• Chemoheterotrophs obtain both energy and carbon from organic molecules. Humans and other animals, fungi, many protists, and many prokaryotes fit this category.

2. Oxygen Use and Energy Production

Prokaryotes differ in whether they require, tolerate, or avoid oxygen:

• Obligate aerobes require oxygen for cellular respiration and cannot live without it.
• Obligate anaerobes do not use oxygen and may be harmed or killed by it. They rely on fermentation or anaerobic respiration.
• Facultative anaerobes use aerobic respiration when oxygen is available but can switch to fermentation or anaerobic respiration when it is not.

The lecture distinguishes two oxygen-free processes:

• Fermentation produces a limited amount of ATP without an electron transport chain. It can generate end products such as ethanol or lactic acid.
• Anaerobic respiration uses an electron transport chain but relies on an electron acceptor other than oxygen, such as a sulfur- or nitrogen-containing compound.

3. Metabolic Cooperation

Prokaryotic cells can cooperate to use resources or perform processes that individual cells could not manage as effectively.

In the cyanobacterium Anabaena, ordinary filament cells carry out photosynthesis, while specialized cells called heterocysts perform nitrogen fixation. Because oxygen produced during photosynthesis would interfere with nitrogen fixation, the processes are separated into different cells.

Photosynthetic cells supply sugars to heterocysts, while heterocysts supply usable nitrogen compounds, such as ammonia, to the other cells. A heterocyst’s thick wall helps limit oxygen from entering.

Biofilms

Biofilms are surface-attached communities of one or more kinds of microorganisms, usually prokaryotes, whose members cooperate metabolically.

• Bacteria secrete polysaccharides and proteins that bind cells together and help the community adhere to a surface.
• Chemical signals can attract nearby cells and encourage the biofilm to grow.
• Channels through the biofilm allow water and nutrients to circulate and wastes to leave.
• Biofilms can cause medical problems by forming on devices such as contact lenses, catheters, ventilators, pacemakers, heart valves, and artificial joints. Dental plaque is also a biofilm.
• Biofilms can be difficult to treat because they may evade immune defenses and resist antibiotics.

4. Bacterial Diversity and Evolutionary Relationships

Research beginning in the 1970s used ribosomal RNA and the DNA that encodes it to compare organisms. This work helped establish the three domains of life: Archaea, Bacteria, and Eukarya.

The lecture describes bacterial evolutionary relationships as still uncertain. Some groups, such as cyanobacteria, are more clearly established, while other relationships remain unresolved. According to the lecture, about 16,000 bacterial and archaeal species had been assigned scientific names. It emphasizes that bacterial species are difficult to define because bacteria do not reproduce sexually and often lack clear morphological distinctions.

A single handful of soil may contain thousands of species, illustrating how much diversity remains poorly understood.

Horizontal Gene Transfer

Horizontal gene transfer is the movement of genes from one genome to another through mechanisms such as plasmids, transposable elements, viruses, and possibly organismal fusion.

The lecture cites a study of 329 bacterial genomes in which an average of 75% of the genes in each genome had been transferred horizontally at some point. Because genes move between lineages, bacterial relationships may look more like a web than a simple branching tree.

5. Medically Important Bacteria Discussed

The lecture profiles several Gram-negative bacteria and related pathogens.

Escherichia coli (E. coli)

• Commonly lives in the lower intestine of mammals and is described as a facultative anaerobe.
• The lecture says it makes up a small portion of the gut community and can benefit its host by producing vitamins, using oxygen that would otherwise affect obligate anaerobes, and competing with harmful bacteria.
• Some strains are pathogenic. The lecture highlights ** E. coli O157:H7**, which is associated with contaminated beef or produce and with fecal contamination of food.

Salmonella

• Causes some forms of food poisoning.
• The lecture identifies raw animal products, including meat and eggs, as sources. It notes that eggshells can be contaminated with fecal matter.

Helicobacter pylori

• The lecture reports that roughly two-thirds of the world’s population is infected, often without symptoms.
• In some people, it causes gastritis or peptic ulcers.
• The lecture also notes an association between infection and increased risk of stomach cancer.

Pseudomonas aeruginosa

• Can cause bloodstream, lung, wound, and other infections, and can grow on medical equipment.
• Drug-resistant strains can be difficult to treat. The lecture discusses healthcare-associated infections and cites estimates of infections and deaths in the United States.
• It also mentions a 2021 study reporting that the bacterium was adapting to conditions in COVID-19 patients.

Neisseria gonorrhoeae

• Causes gonorrhea, a sexually transmitted infection.
• It can infect mucous membranes of the reproductive tract, mouth, throat, eyes, and rectum.
• The lecture cites CDC figures for U.S. infections in 2018.

Chlamydia trachomatis

• An intracellular parasite. The lecture describes it as unusual among Gram-negative bacteria because it lacks peptidoglycan.
• It causes chlamydia, a common sexually transmitted infection that often has no symptoms, and can also cause trachoma, an eye disease.
• In women, infection can spread and cause pelvic inflammatory disease, fallopian-tube damage, infertility, ectopic pregnancy, or chronic pelvic pain.
• The lecture says the CDC recommends yearly screening for sexually active women younger than 25.

Speakers and Sources Featured

• Speaker: One biology lecturer from the UOP Biology channel; no other speaker is identifiable in the subtitles.
• Sources cited or referred to by the lecturer:
  • A biology textbook, used for species estimates and the horizontal gene transfer example.
  • The U.S. Centers for Disease Control and Prevention (CDC), cited for Salmonella and sexually transmitted infection statistics and screening guidance.
  • A 2017 estimate concerning drug-resistant Pseudomonas aeruginosa infections.
  • A 2021 study concerning P. aeruginosa in COVID-19 patients.
  • Ribosomal RNA research beginning in the 1970s and later genome-sequencing research on microbial relationships.