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Bacteria can be classified in a variety of ways. The cell membrane is used in one technique. Christian Gram, a bacteriologist, developed a test in 1884 to detect whether a bacterium possessed a thick, mesh-like membrane called peptidoglycan. Gram positive bacteria have thick peptidoglycan. Gram negative bacteria are those with a thin peptidoglycan coating.

Gram-Positive Bacilli Characteristics

Beyond the peptidoglycan membrane, gram-positive bacilli do not have an exterior cell wall. This increases its absorbency. Its peptidoglycan layer is much thicker than that of gram-negative bacilli. Gram-positive bacilli are rod-shaped. They can also be classified. They can also be characterized based on whether they form spores and whether they need oxygen to survive.  

Types of Gram-Positive Bacilli

Resident flora. Most gram-positive bacilli live harmlessly on your body without causing problems. These are called resident flora. The can be found in the following places on your body:

• Skin
• Nose
• Mouth
• Throat
• Vagina

Some types of gram-positive bacilli are pathogenic, which means they cause diseases in people. Some of the major gram-positive bacteria that cause diseases include:‌

Anthrax. This is a potentially fatal infection that usually affects the skin or lungs but can rarely infect the gastrointestinal tract as well. Anthrax is a spore-forming type of bacilli. It has the potential to be used as a biological weapon because its spores can be spread in the air and be inhaled. The spores can live for decades and are not easily killed.

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Diphtheria. This is a contagious infection of the upper respiratory tract. Vaccination can prevent this infection. It's rare in developed countries.

Enterococcal Infections. These infections can be caused by bacteria known as enterococci. There are over 17 different species. They live in your intestines, where they don't normally cause problems. If they move to another part of your body, such as your blood, heart valves, or skin, they can cause serious infections

Erysipelothricosis. This is a skin infection that usually happens when you get a scrape or puncture wound while handling infected animals. The main symptom is a hard, purplish-red rash.

Listeriosis. This is an infection caused by eating contaminated food. It causes fever, chills, nausea, vomiting, and diarrhea.

Treatment of Gram-Positive Bacilli

Gram-positive bacilli infections are treated with antibiotics. Penicillin, cloxacillin, and erythromycin treat over 90% of gram-positive bacteria.

‌However, antibiotic resistance is becoming a serious problem with gram-positive infections. New drugs are being developed to help with this problem. Antibiotics should only be used when you absolutely need them. Infection control standards need to be followed closely to prevent the development and spread of antibiotic-resistant infections. 

Gram-Negative Bacteria

Gram-negative bacteria have a hard, protective outer shell. Their peptidoglycan layer is much thinner than that of gram-positive bacilli. Gram-negative bacteria are harder to kill because of their harder cell wall. When their cell wall is disturbed, gram-negative bacteria release endotoxins that can make your symptoms worse. 

Gram-negative bacteria can cause many serious infections, including:

• Cholera, a serious intestinal infection
• E. coli
• Plague, an infection of the lymph nodes and lungs
• Cat-scratch disease
• Campylobacter, an infection that usually affects the digestive tract
• Legionnaire's disease, a lung infection
• Salmonella, a digestive infection caused by contaminated food
• Klebsiella, a urinary or respiratory tract infection that usually occurs in long-term care
• Pseudomonas, a group of infections that affect different parts of the body
• Tularemia, an infection that's transmitted by wild animals
• Typhoid fever, an infection that is more common in developing countries

Risks for Developing Gram-Negative Bacterial Infections

Gram-negative infections are most common in hospitals. Your risk increases the longer you stay in the hospital. Other things that increase your risk of developing a gram-negative infection include:

• Undergoing dialysis for kidney disease
• Being on a ventilator
• Having a weak immune system
• Recently having surgery
• Having war wounds
• Using a catheter

How Are Gram-Negative Bacteria Treated?

Gram-negative bacteria have high resistance to antibiotics. They are one of the most serious public health issues in the world. Gram-negative bacteria have the ability to cause a lot of diseases in humans. They can reach almost all of the organ systems. Your doctor may need to try several antibiotics to beat the infection. Older antibiotics may work better. 

Preventing and Controlling Antibiotic Resistance

Antibiotic resistance is made worse by the overuse of antibiotics. You can take the following steps to help control and prevent antibiotic resistance:

• Only use antibiotics prescribed by your healthcare provider.
• Follow your healthcare provider's advice about antibiotic use.
• Don't use or share leftover antibiotics.
• Wash your hands.
• Practice safe sex.
• Keep your vaccines up to date.
• Avoid contact with people who are sick when you can.
• Prepare your food safely.

WebMD Medical Reference