Strains of hVISA and VISA do not have resistant genes found in "Enterococcus" and the proposed mechanisms of resistance include the sequential mutations resulting in a thicker cell wall and the synthesis of excess amounts of D-ala-D-ala residues. VRSA strain acquired the vancomycin resistance gene cluster "vanA" from VRE.

Vancomycin-resistant "Staphylococcus aureus" are strains of "Staphylococcus aureus" that have become resistant to the glycopeptide antibiotic vancomycin.

Vancomycin-resistant "Enterococcus", or vancomycin-resistant enterococci (VRE), are bacterial strains of the genus "Enterococcus" that are resistant to the antibiotic vancomycin.

Once the individual has VRE, it is important to ascertain which "strain".

Multiple drug resistance (MDR), multidrug resistance or multiresistance is antimicrobial resistance shown by a species of microorganism to multiple antimicrobial drugs. The types most threatening to public health are MDR bacteria that resist multiple antibiotics; other types include MDR viruses, fungi, and parasites (resistant to multiple antifungal, antiviral, and antiparasitic drugs of a wide chemical variety). Recognizing different degrees of MDR, the terms extensively drug resistant (XDR) and pandrug-resistant (PDR) have been introduced. The definitions were published in 2011 in the journal "Clinical Microbiology and Infection" and are openly accessible.

Antimicrobial resistance (AMR) is the ability of a microbe to resist the effects of medication previously used to treat them. The term includes the more specific "antibiotic resistance", which applies only to bacteria becoming resistant to antibiotics. Resistant microbes are more difficult to treat, requiring alternative medications or higher doses, both of which may be more expensive or more toxic. Microbes resistant to multiple antimicrobials are called multidrug resistant (MDR); or sometimes superbugs.

Resistance arises through one of three mechanisms: natural resistance in certain types of bacteria, genetic mutation, or by one species acquiring resistance from another. All classes of microbes can develop resistance: fungi develop antifungal resistance, viruses develop antiviral resistance, protozoa develop antiprotozoal resistance, and bacteria develop antibiotic resistance. Resistance can appear spontaneously because of random mutations; or more commonly following gradual buildup over time.

Preventive measures include only using antibiotics when needed, thereby stopping misuse of antibiotics or antimicrobials. Narrow-spectrum antibiotics are preferred over broad-spectrum antibiotics when possible, as effectively and accurately targeting specific organisms is less likely to cause resistance. For people who take these medications at home, education about proper use is essential. Health care providers can minimize spread of resistant infections by use of proper sanitation and hygiene, including handwashing and disinfecting between patients, and should encourage the same of the patient, visitors, and family members.

Rising drug resistance is caused mainly by use of antimicrobials in humans and other animals, and spread of resistant strains between the two. Antibiotics increase selective pressure in bacterial populations, causing vulnerable bacteria to die; this increases the percentage of resistant bacteria which continue growing. With resistance to antibiotics becoming more common there is greater need for alternative treatments. Calls for new antibiotic therapies have been issued, but new drug development is becoming rarer.

Antimicrobial resistance is on the rise. Estimates are that 700,000 to several million deaths result per year. Each year in the United States, at least 2 million people become infected with bacteria that are resistant to antibiotics and at least 23,000 people die as a result. There are public calls for global collective action to address the threat include proposals for international treaties on antimicrobial resistance. Worldwide antibiotic resistance is not fully mapped, but poorer countries with weak healthcare systems are more affected.

Penicillin (PCN or pen) is a group of antibiotics which include penicillin G (intravenous use), penicillin V (use by mouth), procaine penicillin, and benzathine penicillin (intramuscular use). Penicillin antibiotics were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. Penicillins are still widely used today, though many types of bacteria have developed resistance following extensive use.

About 10% of people report that they are allergic to penicillin; however, up to 90% of this group may not actually be allergic. Serious allergies only occur in about 0.03%. All penicillins are β-lactam antibiotics.

Penicillin was discovered in 1928 by Scottish scientist Alexander Fleming. People began using it to treat infections in 1942. There are several enhanced penicillin families which are effective against additional bacteria; these include the antistaphylococcal penicillins, aminopenicillins and the antipseudomonal penicillins. They are derived from "Penicillium" fungi.

Pneumococcal infection is an infection caused by the bacterium "Streptococcus pneumoniae". "S. pneumoniae" is a common member of the bacterial flora colonizing the nose and throat of 5–10% of healthy adults and 20–40% of healthy children. However, it is also the cause of significant disease being a leading cause of pneumonia, bacterial meningitis, and sepsis. The World Health Organization estimate that in 2005 pneumococcal infections were responsible for the death of 1.6 million children worldwide.

Bacteremia (also bacteraemia) is the presence of bacteria in the blood. Blood is normally a sterile environment, so the detection of bacteria in the blood (most commonly accomplished by blood cultures) is always abnormal. It is distinct from sepsis, which is the host response to the bacteria.

Bacteria can enter the bloodstream as a severe complication of infections (like pneumonia or meningitis), during surgery (especially when involving mucous membranes such as the gastrointestinal tract), or due to catheters and other foreign bodies entering the arteries or veins (including during intravenous drug abuse). Transient bacteremia can result after dental procedures or brushing of teeth.

Bacteremia can have several important health consequences. The immune response to the bacteria can cause sepsis and septic shock, which has a high mortality rate. Bacteria can also spread via the blood to other parts of the body (which is called hematogenous spread), causing infections away from the original site of infection, such as endocarditis or osteomyelitis. Treatment for bacteremia is with antibiotics, and prevention with antibiotic prophylaxis can be given in high risk situations.

In humans, "Erysipelothrix rhusiopathiae" infections most commonly present in a mild cutaneous form known as erysipeloid or fish poisoning. "E. rhusiopathiae" can cause an indolent cellulitis, more commonly in individuals who handle fish and raw meat. It gains entry typically by abrasions in the hand. Bacteremia and endocarditis are uncommon but serious sequelae. Due to the rarity of reported human cases, "E. rhusiopathiae" infections are frequently misidentified at presentation.

Increasing bacterial resistance is linked with the volume of antibiotic prescribed, as well as missing doses when taking antibiotics. Inappropriate prescribing of antibiotics has been attributed to a number of causes, including people insisting on antibiotics, physicians prescribing them as they feel they do not have time to explain why they are not necessary, and physicians not knowing when to prescribe antibiotics or being overly cautious for medical and/or legal reasons. Lower antibiotic concentration contributes to the increase of AMR by introducing more mutations that support bacterial growth in higher antibiotic concentration.

For example, sub-inhibitory concentration have induced genetic mutation in bacteria such as "Pseudomonas aeruginosa" and "Bacteroides fragilis".

Up to half of antibiotics used in humans are unnecessary and inappropriate For example, a third of people believe that antibiotics are effective for the common cold, and the common cold is the most common reason antibiotics are prescribed even though antibiotics are useless against viruses. A single regimen of antibiotics even in compliant individuals leads to a greater risk of resistant organisms to that antibiotic in the person for a month to possibly a year.

Antibiotic resistance increases with duration of treatment; therefore, as long as an effective minimum is kept, shorter courses of antibiotics are likely to decrease rates of resistance, reduce cost, and have better outcomes with fewer complications. Short course regimens exist for community-acquired pneumonia spontaneous bacterial peritonitis, suspected lung infections in intense care wards, so-called acute abdomen, middle ear infections, sinusitis and throat infections, and penetrating gut injuries. In some situations a short course may not cure the infection as well as a long course. A BMJ editorial recommended that antibiotics can often be safely stopped 72 hours after symptoms resolve.

Because individuals may feel better before the infection is eradicated, doctors must provide instructions to them so they know when it is safe to stop taking a prescription. Some researchers advocate doctors' using a very short course of antibiotics, reevaluating the patient after a few days, and stopping treatment if there are no clinical signs of infection.

Certain antibiotic classes result in resistance more than others. Increased rates of MRSA infections are seen when using glycopeptides, cephalosporins, and quinolone antibiotics. Cephalosporins, and particularly quinolones and clindamycin, are more likely to produce colonisation with "Clostridium difficile".

Factors within the intensive care unit setting such as mechanical ventilation and multiple underlying diseases also appear to contribute to bacterial resistance. Poor hand hygiene by hospital staff has been associated with the spread of resistant organisms.

The term "penicillin" is often used generically to refer to benzylpenicillin (penicillin G, the original penicillin found in 1928), procaine benzylpenicillin (procaine penicillin), benzathine benzylpenicillin (benzathine penicillin), and phenoxymethylpenicillin (penicillin V). Procaine penicillin and benzathine penicillin have the same antibacterial activity as benzylpenicillin but act for a longer period of time. Phenoxymethylpenicillin is less active against gram-negative bacteria than benzylpenicillin. Benzylpenicillin, procaine penicillin and benzathine penicillin can be given by intravenous or intramuscular injections, but phenoxymethylpenicillin can be given by mouth because of its acidic stability.

Bacteremia is the presence of bacteria in the bloodstream that are alive and capable of reproducing. It is a type of bloodstream infection. Bacteremia is defined as either a primary or secondary process. In primary bacteremia, bacteria have been directly introduced into the bloodstream. Injection drug use may lead to primary bacteremia. In the hospital setting, use of blood vessel catheters contaminated with bacteria may also lead to primary bacteremia. Secondary bacteremia occurs when bacteria have entered the body at another site, such as the cuts in the skin, or the mucous membranes of the lungs (respiratory tract), mouth or intestines (gastrointestinal tract), bladder (urinary tract), or genitals. Bacteria that have infected the body at these sites may then spread into the lymphatic system and gain access to the bloodstream, where further spread can occur.

Bacteremia may also be defined by the timing of bacteria presence in the bloodstream: transient, intermittent, or persistent. In transient bacteremia, bacteria are present in the bloodstream for minutes to a few hours before being cleared from the body, and the result is typically harmless in healthy people. This can occur after manipulation of parts of the body normally colonized by bacteria, such as the mucosal surfaces of the mouth during teeth brushing, flossing, or dental procedures, or instrumentation of the bladder or colon. Intermittent bacteremia is characterized by periodic seeding of the same bacteria into the bloodstream by an existing infection elsewhere in the body, such as an abscess, pneumonia, or bone infection, followed by clearing of that bacteria from the bloodstream. This cycle will often repeat until the existing infection is successfully treated. Persistent bacteremia is characterized by the continuous presence of bacteria in the bloodstream. It is usually the result of an infected heart valve, a central line-associated bloodstream infection (CLABSI), an infected blood clot (suppurative thrombophlebitis), or an infected blood vessel graft. Persistent bacteremia can also occur as part of the infection process of typhoid fever, brucellosis, and bacterial meningitis. Left untreated, conditions causing persistent bacteremia can be potentially fatal.

Bacteremia is clinically distinct from sepsis, which is a condition where the blood stream infection is associated with an inflammatory response from the body, often causing abnormalities in body temperature, heart rate, breathing rate, blood pressure, and white blood cell count.

New or progressive infiltrate on the chest X-ray with one of the following:

- Fever > 37.8 °C (100 °F)

- Purulent sputum

- Leukocytosis > 10,000 cells/μl

In an elderly person, the first sign of hospital-acquired pneumonia may be mental changes or confusion.

Other symptoms may include:

- A cough with greenish or pus-like phlegm (sputum)

- Fever and chills

- General discomfort, uneasiness, or ill feeling (malaise)

- Loss of appetite

- Nausea and vomiting

- Sharp chest pain that gets worse with deep breathing or coughing

- Shortness of breath

- Decreased blood pressure and fast heart rate

Drug resistance is the reduction in effectiveness of a medication such as an antimicrobial or an antineoplastic in curing a disease or condition. The term is used in the context of resistance that pathogens or cancers have "acquired", that is, resistance has evolved. Antimicrobial resistance and antineoplastic resistance challenge clinical care and drive research. When an organism is resistant to more than one drug, it is said to be multidrug-resistant. Even the immune system of an organism is in essence a drug delivery system, albeit endogenous, and faces the same arms race problems as external drug delivery.

The development of antibiotic resistance in particular stems from the drugs targeting only specific bacterial molecules (almost always proteins). Because the drug is "so" specific, any mutation in these molecules will interfere with or negate its destructive effect, resulting in antibiotic resistance. Furthermore there is mounting concern over the abuse of antibiotics in the farming of livestock, which in the European Union alone accounts for three times the volume dispensed to humans – leading to development of super-resistant bacteria.

Bacteria are capable of not only altering the enzyme targeted by antibiotics, but also by the use of enzymes to modify the antibiotic itself and thus neutralise it. Examples of target-altering pathogens are "Staphylococcus aureus", vancomycin-resistant enterococci and macrolide-resistant "Streptococcus", while examples of antibiotic-modifying microbes are "Pseudomonas aeruginosa" and aminoglycoside-resistant "Acinetobacter baumannii".

In short, the lack of concerted effort by governments and the pharmaceutical industry, together with the innate capacity of microbes to develop resistance at a rate that outpaces development of new drugs, suggests that existing strategies for developing viable, long-term anti-microbial therapies are ultimately doomed to failure. Without alternative strategies, the acquisition of drug resistance by pathogenic microorganisms looms as possibly one of the most significant public health threats facing humanity in the 21st century.

Resistance to chemicals is only one aspect of the problem, another being resistance to physical factors such as temperature, pressure, sound, radiation and magnetism, and not discussed in this article, but found at Physical factors affecting microbial life.

"S. pneumoniae" is responsible for 15–50% of all episodes of community acquired pneumonia, 30–50% of all cases of acute otitis media, and a significant proportion of bloodstream infections and bacterial meningitis.

As estimated by WHO in 2005 it killed about 1.6 million children every year worldwide with 0.7–1 million of them being under the age of five. The majority of these deaths were in developing countries.

Common multidrug-resistant organisms are usually bacteria:

- Vancomycin-Resistant Enterococci (VRE)

- Methicillin-Resistant "Staphylococcus" "aureus" (MRSA)

- Extended-spectrum β-lactamase (ESBLs) producing Gram-negative bacteria

- "Klebsiella" "pneumoniae" carbapenemase (KPC) producing Gram-negatives

- Multidrug-Resistant gram negative rods (MDR GNR) MDRGN bacteria such as "Enterobacter species", "E.coli", "Klebsiella pneumoniae", "Acinetobacter baumannii", "Pseudomonas aeruginosa"

A group of gram-positive and gram-negative bacteria of particular recent importance have been dubbed as the ESKAPE group ("Enterococcus faecium", "Staphylococcus aureus", "Klebsiella pneumoniae", "Acinetobacter baumannii", "Pseudomonas aeruginosa" and Enterobacter species).

- Multi-drug-resistant tuberculosis

"Staphylococcus" is a genus of Gram-positive bacteria that can cause a wide variety of infections in humans and other animals through infection or the production of toxins.

Staphylococcal toxins are a common cause of food poisoning, as they can be produced in improperly-stored food. Staphylococci are also known to be a cause of bacterial conjunctivitis. "Staphylococcus aureus" can cause a number of different skin diseases. Among neurosurgical patients, it can cause community-acquired meningitis.

The treatment of choice is a single dose of benzathine benzylpenicillin given by intramuscular injection, or a five-day to one-week course of either oral penicillin or intramuscular procaine benzylpenicillin. Erythromycin or doxycycline may be given instead to people who are allergic to penicillin. "E. rhusiopathiae" is intrinsically resistant to vancomycin.

Hospital-acquired pneumonia (HAP) or nosocomial pneumonia refers to any pneumonia contracted by a patient in a hospital at least 48–72 hours after being admitted. It is thus distinguished from community-acquired pneumonia. It is usually caused by a bacterial infection, rather than a virus.

HAP is the second most common nosocomial infection (after urinary tract infections) and accounts for 15–20% of the total. It is the most common cause of death among nosocomial infections and is the primary cause of death in intensive care units.

HAP typically lengthens a hospital stay by 1–2 weeks.

Boils are bumpy, red, pus-filled lumps around a hair follicle that are tender, warm, and very painful. They range from pea-sized to golf ball-sized. A yellow or white point at the center of the lump can be seen when the boil is ready to drain or discharge pus. In a severe infection, an individual may experience fever, swollen lymph nodes, and fatigue. A recurring boil is called chronic furunculosis. Skin infections tend to be recurrent in many patients and often spread to other family members. Systemic factors that lower resistance commonly are detectable, including: diabetes, obesity, and hematologic disorders. Boils can be caused by other skin conditions that cause the person to scratch and damage the skin.

Boils may appear on the buttocks or near the anus, the back, the neck, the stomach, the chest, the arms or legs, or even in the ear canal. Boils may also appear around the eye, where they are called styes. A boil on the gum is called intraoral dental sinus, or more commonly, a gumboil.

The generic name Staphylococcus is derived from the Greek word "staphyle," meaning bunch of grapes, and "kokkos," meaning granule. The bacteria, when seen under a microscope, appear like a branch of grapes or berries.

A carbuncle is a cluster of several boils, which is typically filled with purulent exudate (dead neutrophils, phagocitized bacteria, & other cellular components). Fluid may drain freely from the carbuncle, or intervention involving an incision and drainage procedure may be needed. Carbuncles may develop anywhere, but they are most common on the back and the nape of the neck.

A carbuncle is palpable and can range in size to be as small as a pea or as large as a golf ball. As the impending infection develops, itching may occur. There may be localized erythema, skin irritation, and the area may be painful when touched. Sometimes more severe symptoms may occur, such as fatigue, fever, chills, and malaise.

A hospital-acquired infection (HAI), also known as a nosocomial infection, is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is sometimes instead called a health care–associated infection (HAI or HCAI). Such an infection can be acquired in hospital, nursing home, rehabilitation facility, outpatient clinic, or other clinical settings. Infection is spread to the susceptible patient in the clinical setting by various means. Health care staff can spread infection, in addition to contaminated equipment, bed linens, or air droplets. The infection can originate from the outside environment, another infected patient, staff that may be infected, or in some cases, the source of the infection cannot be determined. In some cases the microorganism originates from the patient's own skin microbiota, becoming opportunistic after surgery or other procedures that compromise the protective skin barrier. Though the patient may have contracted the infection from their own skin, the infection is still considered nosocomial since it develops in the health care setting.

In the United States, the Centers for Disease Control and Prevention estimated roughly 1.7 million hospital-associated infections, from all types of microorganisms, including bacteria and fungi combined, cause or contribute to 99,000 deaths each year. In Europe, where hospital surveys have been conducted, the category of gram-negative infections are estimated to account for two-thirds of the 25,000 deaths each year. Nosocomial infections can cause severe pneumonia and infections of the urinary tract, bloodstream and other parts of the body. Many types are difficult to treat with antibiotics. In addition, antibiotic resistance can complicate treatment.

The initial cause of a carbuncle can often not be determined. Triggers that make carbuncle infections more likely include recent incidence of folliculitis; friction from clothing or shaving; having hair pulled out, such as sites where clothing or furniture grab at hairs; generally poor hygiene; poor nutrition; or weakened immunity. Poor health may be a predisposing factor – for example, persons with diabetes and immune system diseases are more likely to develop infections (especially bacterial infections of the leg or foot).