Bacteria have long been classified according to how they are stained by the chemical dye called the Gram stain. Pseudomonas aeruginosa is Gram-negative, compared to the Staph aureus which is gram-positive.
Being Gram negative, Pseudomonas has an extra membrane, the outer membrane, and a shell of a nasty material called Lipo polysaccharide. These extra structures act as a barrier to entrance of antibiotics. In addition, the Pseudomonas has many more genes than the average bacterium and uses these genes for adaptation. For instance, it is good at forming a raft of mutually supporting cells called a biofilm, which enables it to become particularly entrenched. In a condition of the lung called cystic fibrosis, this biofilm contains specialized cells, one of which is called the Persister cell. When Pseudomonas senses that a critical number of bacteria is present, called quorum sensing, the persister cell multiplies to become around 10% of the bacterial population, and slows down their metabolism massively, becoming a sort of “zombie cell”. These cells are very hard to kill and persist through an antibiotic treatment that kills other Pseudomonas cells, only to rev up their metabolism and become active again once treatment is withdrawn.
In addition, Pseudomonas has all of the other resistant talents mentioned in the previous article on Staphylococcus aureus, such as plasmid acceptance, ability to destroy penicillin, efflux pumps, and rerouting of metabolism.
In trying to control Pseudomonas, techniques other than antibiotics are being tried out of desperation, including interference with a quorum sensing, use of bacteriophages and chemical elements such as Gallium which masquerades as the iron this bacterium requires.
Pseudomonas is not as actively pathogenic as Staphylococcus aureus, but it has made a great niche for itself in the respiratory tract, especially in people with a compromised immune system, or pulmonary abnormality such as cystic fibrosis. Ventilation tubes and other hospital equipment can become contaminated and spread the infection, unless thoroughly cleansed of Pseudomonas; it is very sensitive to acidic solutions, and those who need nasal CPAP for sleep apnea may recall that they have to rinse their equipment in vinegar, or acetic acid. Swimmers can get an external ear infection with this organism, and the drops for swimmers ear often contains acetic acid.
The next rogue to consider is a frightful yeast called Candida auris.