Difference Between Gram-Positive and Gram-Negative Bacillus

Medically Reviewed by Sabrina Felson, MD on May 23, 2023
4 min read

There are many ways of classifying bacteria. One method is based on the cell membrane. In 1884, a bacteriologist named Christian Gram created a test that could determine if a bacterium had a thick, mesh-like surrounding cell membrane called peptidoglycan, which is composed of sugars and amino acids. Bacteria with thick peptidoglycan are called gram positive. If the peptidoglycan layer is thin, it's classified as gram negative.

A gram-positive bacillus doesn't have an outer cell wall beyond the peptidoglycan membrane. This makes it more absorbent. Its peptidoglycan layer is much thicker than the peptidoglycan layer on gram-negative bacilli.  Gram-positive bacilli are shaped like rods. They can also be characterized based on whether they form spores and whether they need oxygen to survive.  

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.

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.

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 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:

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:

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. 

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 health care 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.
  • Choose meat from animals that haven't been fed antibiotics.
  • Prepare your food safely.