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Infectious Diseases

Infectious Disease information and resources for the MSK community, including clinicians, patients, and the general public.

What are Bacteria?

Bacteria are living things that have only one cell. Under a microscope, they look like balls, rods, or spirals. They are so small that a line of 1,000 could fit across a pencil eraser. Most types of don't make you sick. Many types are helpful. Some of them help to digest food, destroy disease-causing cells, and give the body needed vitamins. Bacteria are also used in making healthy foods like yogurt and cheese.

But infectious bacteria can make you ill. They reproduce quickly in your body. Many give off chemicals called toxins, which can damage tissue and make you sick.

Antibiotics are the usual treatment. When you take antibiotics, follow the directions carefully. Each time you take antibiotics, you increase the chances that bacteria in your body will learn to resist them causing antibiotic resistance. Later, you could get or spread an infection that those antibiotics cannot cure.

Gram-Positive Bacteria

Gram-positive bacteria are bacteria classified by the color they turn in the staining method. The staining method uses crystal violet dye, which is retained by the thick peptidoglycan cell wall found in gram-positive organisms. This reaction gives gram-positive organisms a blue color when viewed under a microscope.

Other information used to differentiate bacteria is the shape: cocci (clusters or chains) and bacilli (rods).

Gram-positive cocci:
  • Staphylococcus
  • Streptococcus
  • Enterococci
Gram-positive bacilli:
  • Bacillus anthracis
  • Bacillus cereus
  • Clostridioides difficile
  • Listeria monocytogenes
  • Corynebacterium diphtheria
  • Mycobacterium tuberculosis

Gram-Negative Bacteria

Gram-negative bacteria (GNB) are among the world's most significant public health problems due to their high resistance to antibiotics. These microorganisms have significant clinical importance in hospitals because they often require patients to be in the intensive care unit (ICU), and patients are at high risk of morbidity and mortality


Enterobacteriaceae are a large and very heterogenous family of GNB (over 100 species) causing a wide variety of diseases, accounting for approximately 80% of gram-negative isolates in humans. Enterobacteriaceae is often referred to as enterobacteria or "enteric bacteria", as they are found in the normal gut microbia of many animals or are parasites on animals or plants.

The most common disease-causing species are:

  • Escherichia
  • Salmonella
  • Klebsiella
  • Shigella

In a clinical setting, three species make up 80 to 95% of all isolates identified: Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis.

There are several strains of Enterobacteriaceae that isolates have been found that are resistant to carbapenems, which are often claimed as "the last line of antibiotic defense" against resistant organisms. Carbapenem-resistant Enterobacterales (CRE) include both Klebsiella pneumoniae and Escherichia coli.


The non-fermenter, gram-negative bacilli (BNF) have a lower frequency of isolation when compared to Enterobacteriaceae; however, they cause severe, fatal infections, especially in the hospital environment. They also cause opportunistic diseases in ICU patients who undergo invasive procedures.

The main BNF microorganisms that cause human disease are:

  •  Pseudomonas aeruginosa
  • Acinetobacter baumannii
  • Burkholderia cepacia
  • Burkholderia pseudomallei
  • Stenotrophomonas
  • Alcaligenes
  • Moraxella

Other Gram Negative Bacteria
  • Neisseria
  • Haemophilus spp.
  • Helicobacter pylori
  • Chlamydia trachomatis (Chlamydia)
  • Bordetella pertussis (Whooping Cough)