Gram-negative bacteria

Some tiny germs have two skins.

These skins act like a shield. They keep out bad things. This shield stops some medicines from working. It also stops soaps from hurting them.

These germs live everywhere on Earth. They can be found in many places. Some of these germs make people sick.

They can cause problems in our bodies.

One part of the skin can be dangerous. If it gets into our blood, it can make us feel very ill. It can cause a fever. It can also make it hard to breathe.

Doctors use special tools to find these germs. They can see how the germs react to dyes. This helps them know how to help. It is a way to stay safe.

Some tiny germs have a special build. We call them Gram-negative bacteria.

Unlike other germs, these have two skins. We call these skins membranes. They have an inner membrane and an outer membrane. A thin layer called peptidoglycan sits between them.

This layer is much thinner than in other bacteria.

These two skins act like a strong shield. They help the germ stay safe. This shield blocks many medicines, like penicillin. It also stops soaps and natural body cleaners from hurting them. The space between the skins is filled with a gel. This is called the periplasm. It holds parts that can break down medicines.

One part of the outer skin is very complex. It contains a substance called lipopolysaccharide, or LPS.

If these germs get into your blood, the LPS can be dangerous. It can trigger a reaction in your body. This can lead to septic shock. This is a very serious state. It can cause low blood pressure and trouble breathing. Doctors must work fast to treat these infections.

Gram-negative bacteria are tiny living things found in almost every place on Earth. They are known by how they react to a special purple dye called crystal violet. When scientists use a method called Gram staining, these bacteria do not hold onto the purple color.

Instead, they appear different under a microscope than Gram-positive bacteria. This difference is very important for doctors and scientists to understand. It helps them identify which germs are causing an illness. Knowing the type of bacteria can change how a person is treated.

These bacteria have a very special way they are built. Most bacteria have one skin, but these have two membranes.

They have an inner membrane and an outer membrane. A thin layer called peptidoglycan sits like a sandwich between them. The space between these two layers is filled with a gel-like substance called the periplasm. This space also holds special enzymes that can break down medicines. The outer membrane also has tiny holes called porins that let certain molecules pass through.

Scientists have studied these bacteria for a long time to group them. In the past, people used the Gram stain to divide all bacteria into four main groups. One group was called the Gracillicutes.

Since 1987, new studies using molecules have shown that these groups are more complex. We now know that the bacteria with a specific outer layer belong to a group called Pseudomonadati. Other bacteria, like the Mycobacteriales, also have an outer membrane but use a different material called mycolic acid. This shows that nature found different ways to build similar shields.

There are many different types of these bacteria in our world. Some are very common, like Escherichia coli, which is often used in science labs.

Others can cause serious sickness in people. For example, Pseudomonas aeruginosa can cause lung problems. Salmonella and Helicobacter pylori can cause problems in the stomach. Some bacteria, like Neisseria meningitidis, can cause meningitis. Doctors use many different medicines to fight them, such as aminoglycosides or carbapenems.

The double membrane acts like a very strong suit of armor. This shield protects the bacteria from many things. It can block medicines like penicillin from working. It also stops detergents and natural body cleaners from hurting the cell. One part of the outer layer is called lipopolysaccharide, or LPS. If these bacteria enter the blood, the LPS can cause a very dangerous reaction. This can lead to septic shock, which causes low blood pressure and trouble breathing. This is why these bacteria are such a big focus for doctors.

Gram-negative bacteria are a diverse group of microscopic organisms found in nearly every environment on Earth that supports life. They are primarily defined by their unique cell envelope structure, which distinguishes them from Gram-positive bacteria during a laboratory procedure called Gram staining. When scientists apply a crystal violet dye to a sample, Gram-negative bacteria do not retain the purple color.

Instead, they appear differently under a microscope. This distinction is vital for medical professionals because the structural differences between these groups dictate how they interact with the environment and how they respond to medical treatments.

The defining characteristic of these bacteria is their "diderm" structure, meaning they possess two distinct lipid membranes.

An inner cytoplasmic membrane surrounds the cell's interior. Sandwiched between this inner layer and an outer membrane is a thin layer of peptidoglycan, which provides structural support. The space between these two membranes is filled with a concentrated, gel-like substance known as the periplasm. This periplasmic space is significant because it contains enzymes that can modify or break down antibiotics. Additionally, the outer membrane contains specialized proteins called porins, which act like pores to allow specific molecules to pass through the protective barrier.

One of the most complex parts of the Gram-negative envelope is the outer leaflet of the outer membrane. This layer contains a large molecule called lipopolysaccharide, or LPS. The LPS structure is composed of three specific parts: lipid A, a core polysaccharide, and an O antigen.

The lipid A component is particularly important in medicine because it functions as an endotoxin. When immune cells attack and break apart (lyse) these bacteria, the released LPS can trigger a massive, toxic reaction in the host. This reaction may lead to septic shock, a dangerous condition characterized by low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis.

Because of this double-membrane shield, Gram-negative bacteria present significant challenges in the medical field. The outer membrane acts as a highly effective barrier against many substances that would easily kill other bacteria. It provides resistance to detergents and the antimicrobial enzyme lysozyme, which is produced by animals as part of their innate immune system. Furthermore, the membrane blocks many common antibiotics, such as penicillin. To combat these organisms, scientists have developed several classes of drugs. These include aminoglycosides, monobactams like aztreonam, and carbapenems. Other treatments include quinolones, cephalosporins, and combinations like piperacillin-tazobactam.

Historically, the classification of bacteria relied heavily on the Gram stain. In the past, the kingdom Monera was divided into four groups based on this staining response: Firmicutes, Gracillicutes, Mollicutes, and Mendocutes. However, molecular studies since 1987 have disproven the idea that all Gram-negative bacteria belong to a single evolutionary lineage. We now know that the "conventional" Gram-negative bacteria, which possess the LPS outer membrane, share a common ancestor and belong to the kingdom Pseudomonadati. Other bacteria, such as the order Mycobacteriales, have also evolved an outer membrane, but they use mycolic acid instead of LPS. This means that having an outer membrane is not a trait exclusive to one single family tree.

There are many notable species within the Gram-negative category that impact human health and science. The proteobacteria superphylum includes the well-known Escherichia coli, which is often used as a model organism in laboratories.

Other species in this group include Salmonella and various Enterobacteriaceae. Some Gram-negative bacteria are specifically known for causing respiratory issues, such as Legionella pneumophila or Pseudomonas aeruginosa. Others are linked to urinary tract problems, like Proteus mirabilis, or gastrointestinal issues, such as Helicobacter pylori. In hospital settings, species like Acinetobacter baumannii are particularly concerning because they can cause serious infections like bacteremia and meningitis.

Understanding these bacteria also involves studying how they exchange genetic information through a process called horizontal gene transfer. One method is transformation, where a bacterium takes up foreign genetic material from its surrounding medium. As of 2014, approximately 80 species of bacteria were known to be capable of transformation, split somewhat evenly between Gram-positive and Gram-negative types. This process has been studied in many medically important species, including Neisseria meningitidis and Vibrio cholerae. By studying how these bacteria change and adapt, scientists gain a deeper understanding of how they evolve resistance and survive in complex biological systems.