Bacteria are small single celled organisms that live all around us, from the harshest environments (e.g. deep sea thermal vents) to our skin. They are quite different to human cells and are referred to as "prokaryotes". Prokaryotes are a subset of living things, the other subsets being eukaryotes and archaea. - Prokaryotes are simple and single celled, they lack both a nucleus and other membrane bound organelles - Eukaryotes can be single or multi celled, they contain a nucleus and other complex organelles. - Archaea are similar to prokaryotes and are sometimes put under the same heading, however they are also evolutionarily linked to eukaryotes, they are single celled and don't contain a nucleus but their biochemistry makes them distinct. > **Fun Fact- Viruses** > Viruses are not considered to be living things, although we often talk about viruses and bacteria when discussing disease; viruses are distinct. So then, **what is a virus?** A virus is a sub microscopic disease causing agent that can only replicate inside living cells (including bacteria!). they use the infected cells own machinery to produce more viruses. **Why aren't they considered living?** A simple answer would be that they don't fulfil our requirements of living; **MRS GREN** is an acronym commonly used to explain simply what it means to be alive **Movement**, **Respiration**, **Sensation**, **Growth**, **reproduction**, **Excretion** and **Nutrition**. although it is more complicated than that in reality, its a good starting point. Viruses meet basically none of our requirements of being alive they are literally just genetic material packaged up- even for reproduction they have to hijack a hosts machinery. Bacteria come in many shapes, sizes, and are found in various environmental niches, but one of the simplest and most widely used ways to group them is by the **Gram stain**, Gram staining is a technique developed in 1884 by Hans Christian Gram. Originally, Hans was trying to make bacteria in lung tissue more visible under a microscope. He observed that some bacteria kept the purple dye even after being washed with alcohol, while others lost the colour and took up a different counterstain instead. This observation turned into a fundamental microbiology classification: **Gram-positive** and **Gram-negative**. - The name doesn’t describe where they live or what diseases they can cause, it only describes how their cell envelopes (the structures surrounding the cell) interact with dyes in the Gram staining process. - In practice, Gram staining is still used today in labs to help quickly narrow down the identity of a pathogen and choose appropriate antibiotics. **How the categories work:** - **Gram-positive bacteria** such as *Staphylococcus aureus* have a relatively simple envelope: a single membrane surrounded by a thick, jelly-like layer of **peptidoglycan** (PG), about 20–80 nm thick. This peptidoglycan traps the crystal violet dye, so after washing and counterstaining, they appear **purple** under the microscope. - **Gram-negative bacteria** such as *Escherichia coli* have a more complex, **three-layered envelope**: an **inner membrane**, a thin peptidoglycan layer, and then an **outer membrane**. During staining, the alcohol wash disrupts the outer membrane and allows the crystal violet to escape, so when they’re counterstained (often with **safranin**), they appear **red or pink**. While some bacteria, like _Ureaplasma urealyticum_, don’t fit neatly into either group, the Gram classification remains extremely useful. The structure of the cell envelope has huge implications for how bacteria interact with the environment, resist antibiotics, and cause disease. Gram-negative bacteria, in particular, pose a serious global health risk because their outer membrane gives them extra protection and resilience against many antibiotics and disinfectants. Understanding the unique properties of this outer membrane is key to developing new treatments. # So, what is a membrane? A cell membrane (AKA plasma membrane, lipid bilayer, or simply membrane) is a thin layer that covers and coats all cells. It is also is responsible for enclosing certain organelles such as the nuclei or mitochondria. It functions to both protect and control what goes in and out of a cell but also "compartmentalise" (separate apart) different areas inside of cells. When thinking about bacteria the only membranes they have are the ones that enclose them. The structure of a membrane is generally referred to as a lipid-bilayer, although you might see phospholipid bilayer. It is important to highlight here that while most membrane found across the domains of life are phospholipid bilayers; when it comes to gram-negative bacteria their outer membrane is different and is an asymmetrical bilayer made of phospholipids on one side and Lipopolysaccharide on another! I will explain this more below :D ### **What it means it means to be a lipid bilayer** ![[20250812_160918.png|400]] Lipids are biological molecules made of two main parts: - a **polar head group** containing glycerol and phosphate, which is **hydrophilic** (“water-loving”) and interacts happily with watery environments, and - **fatty acid tails**, which are **hydrophobic** (“water-fearing”) and avoid contact with water. When placed in aqueous environments, lipids naturally arrange themselves so that their hydrophilic heads face the surrounding water, while their hydrophobic tails tuck away from it. This results in a **bilayer**: two layers of lipids aligned tail-to-tail, forming a stable barrier. It should be said that this self assembly is less applicable when thinking about bacteria in isolation, the lipids are guided by protein pathways that are highly regulated to make sure that they end up in the right place and where they need to go. In most biological membranes, both layers of the bilayer are made of **phospholipids**. However, in the _outer membrane_ of Gram-negative bacteria, the two layers are **asymmetrical** the inner leaflet is phospholipids, but the outer leaflet is mostly **lipopolysaccharide (LPS)**. This difference in composition gives the outer membrane unique properties and is key to its role in protecting the cell. LPS interact more strongly with each other than phospholipids do which is why it provides these unique qualities ![[Pasted image 20250812163201.png|400]] https://www.nature.com/articles/srep38610