Anaerobes can be isolated from most types of upper respiratory tract and head and neck and infection and are especially common in chronic ones. These include tonsillar, peritonsillar and retropharyngeal abscesses, chronic otitis media, sinusitis and mastoiditis, eye ocular) infections, all deep neck space infections, parotitis, sialadenitis, thyroiditis, odontogenic infections, and postsurgical and nonsurgical head and neck wounds and abscesses., The predominant organisms are of oropharyngeal flora origin and include AGNB, "Fusobacterium" and Peptostreptococcus spp.

Anaerobes involve almost all dental infections. These include dental abscesses, endodontal pulpitis and periodontal (gingivitis and periodontitis) infections, and perimandibular space infection. Pulpitis can lead to abscess formation and eventually spread to the mandible and other neck spaces. In addition to strict anaerobic bacteria, microaerophilic streptococci and "Streptococcus salivarius" can also be present.

"Fusobacterium" spp. and anaerobic spirochetes are often the cause of acute necrotizing ulcerative gingivitis (or Vincent's angina) which is a distinct form of ulcerative gingivitis.

Deep neck infections that develop as a consequence of oral, dental and pharyngeal infections are generally polymicrobial in nature. These include extension of retropharyngeal cellulitis or abscess, mediastinitis following esophagus perforation, and dental or periodontal abscess.

Secondary peritonitis and intra-abdominal abscesses including splenic and hepatic abscesses generally occur because of the entry of enteric micro-organisms into the peritoneal cavity through a defect in the wall of the intestine or other viscus as a result of obstruction, infarction or direct trauma. Perforated appendicitis, diverticulitis, inflammatory bowel disease with perforation and gastrointestinal surgery are often associated with polymicrobial infections caused by aerobic and anaerobic bacteria, where the number of isolates can average 12 (two-thirds are generally anaerobes). The most common aerobic and facultative bacteria are "Escherichia coli", "Streptococcus" spp. (including Enterococcus spp.), and the most frequently isolated anaerobic bacteria are the "B. fragilis" group, "Peptostreptococcus" spp., and "Clostridium" spp.

Abdominal infections are characteristically biphasic: an initial stages of generalized peritonitis associated with "Escherichia coli" sepsis, and a later stages, in which intra abdominal abscesses harboring anaerobic bacteria ( including "B. fragilis" group ) emerge.

The clinical manifestations of secondary peritonitis are a reflection of the underlying disease process. Fever, diffuse abdominal pain, nausea and vomiting are common. Physical examination generally show signs of peritoneal inflammation, isuch as rebound tenderness, abdominal wall rigidity and decrease in bowel sounds. These early findings may be followed by signs and symptoms of shock.

Biliary tract infection is usually caused by "E. coli, Klebsiella" and "Enterococcus" spp. Anaerobes (mostly "B. fragilis" group, and rarely "C. perfringens") can be recovered in complicated infections associated with carcinoma, recurrent infection, obstruction, bile tract surgery or manipulation.

Laboratory studies show elevated blood leukocyte count and predominance of polymorphonuclear forms. Radiographs studies may show free air in the peritoneal cavity, evidence of ileus or obstruction and obliteration of the psoas shadow. Diagnostic ultrasound, gallium and CT scanning may detect appendiceal or other intra-abdominal abscesses. Polymicrobial postoperative wound infections can occur.

Treatment of mixed aerobic and anaerobic abdominal infections requires the utilization of antimicrobials effective against both components of the infection as well as surgical correction and drainage of pus. Single and easily accessible abscesses can be drained percutaneously.

Gram-negative bacterial infection refers to a disease caused by gram-negative bacteria. One example is E. coli.

It is important to recognize that this class is defined morphologically (by the presence of a bacterial outer membrane), and not histologically (by a pink appearance when stained), though the two usually coincide.

One reason for this division is that the outer membrane is of major clinical significance: it can play a role in the reduced effectiveness of certain antibiotics, and it is the source of endotoxin.

The gram status of some organisms is complex or disputed:

- Mycoplasma are sometimes considered gram-negative, but because of its lack of a cell wall and unusual membrane composition, it is sometimes considered separately from other gram-negative bacteria.

- Gardnerella is often considered gram-negative, but it is classified in MeSH as both gram-positive and gram-negative. It has some traits of gram-positive bacteria, but has a gram-negative appearance. It has been described as a "gram-variable rod".

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.

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.

The several forms of the infection are:

- Skin/subcutaneous tissue disease is a septic phlegmon that develops classically in the hand and forearm after a cat bite. Inflammatory signs are very rapid to develop; in 1 or 2 hours, edema, severe pain, and serosanguineous exudate appear. Fever, moderate or very high, can be seen, along with vomiting, headache, and diarrhea. Lymphangitis is common. Complications are possible, in the form of septic arthritis, osteitis, or evolution to chronicity.

- Sepsis is very rare, but can be as fulminant as septicaemic plague, with high fever, rigors, and vomiting, followed by shock and coagulopathy.

- Pneumonia disease is also rare and appears in patients with some chronic pulmonary pathology. It usually presents as bilateral consolidating pneumonia, sometimes very severe.

- Zoonosis, pasteurellosis can be transmitted to humans through cats.

Other locations are possible, such as septic arthritis, meningitis, and acute endocarditis, but are very rare.

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

Diagnosis is made with isolation of "Pasteurella multocida" in a normally sterile site (blood, pus, or cerebrospinal fluid).

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.

"Klebsiella" pneumonia (KP) is a form of bacterial pneumonia associated with "Klebsiella pneumoniae". It is typically due to aspiration and alcoholism may be a risk factor, though it is also commonly implicated in hospital-acquired urinary tract infections, and COPD(chronic obstructive pulmonary disease) individuals

The disease is characterised by the formation of painful abscesses in the mouth, lungs, breast, or gastrointestinal tract. Actinomycosis abscesses grow larger as the disease progresses, often over months. In severe cases, they may penetrate the surrounding bone and muscle to the skin, where they break open and leak large amounts of pus, which often contains characteristic granules (sulfur granules) filled with progeny bacteria. These granules are named due to their appearance, but are not actually composed of sulfur.

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.

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.

"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.

Carbapenem-resistant Enterobacteriaceae (CRE) have been defined as carbapenem-nonsusceptible and extended-spectrum cephalosporin-resistant "Escherichia coli, Enterobacter aerogenes, Enterobacter cloacae" complex, "Klebsiella pneumoniae", or "Klebsiella oxytoca". Some exclude ertapenem resistance from the definition.

"Actinomycosis" is a rare infectious bacterial disease caused by "Actinomyces" species. About 70% of infections are due to either "Actinomyces israelii" or "A. gerencseriae". Infection can also be caused by other "Actinomyces" species, as well as "Propionibacterium propionicus", which presents similar symptoms. The condition is likely to be polymicrobial aerobic anaerobic infection.

"Streptococcus pneumoniae" () is the most common bacterial cause of pneumonia in all age groups except newborn infants. "Streptococcus pneumoniae" is a Gram-positive bacterium that often lives in the throat of people who do not have pneumonia.

Other important Gram-positive causes of pneumonia are "Staphylococcus aureus" () and "Bacillus anthracis".

Bacterial pneumonia is a type of pneumonia caused by bacterial infection.

The signs and symptoms of Lemierre's syndrome vary, but usually start with a sore throat, fever, and general body weakness. These are followed by extreme lethargy, spiked fevers, rigors, swollen cervical lymph nodes, and a swollen, tender or painful neck. Often there is abdominal pain, diarrhea, nausea and vomiting during this phase. These signs and symptoms usually occur several days to 2 weeks after the initial symptoms.

Symptoms of pulmonary involvement can be shortness of breath, cough and painful breathing (pleuritic chest pain). Rarely, blood is coughed up. Painful or inflamed joints can occur when the joints are involved.

Septic shock can also arise. This presents with low blood pressure, increased heart rate, decreased urine output and an increased rate of breathing. Some cases will also present with meningitis, which will typically manifest as neck stiffness, headache and sensitivity of the eyes to light.

Liver enlargement and spleen enlargement can be found, but are not always associated with liver or spleen abscesses.

Other signs and symptoms that may occur:

- Headache (unrelated to meningitis)

- Memory loss

- Muscle pain

- Jaundice

- Decreased ability to open the jaw

- Crepitations are sometimes heard over the lungs

- Pericardial friction rubs as a sign of pericarditis (rare)

- Cranial nerve paralysis and Horner's syndrome (both rare)

In documented BPF cases, the symptoms include high fever (101.3 degrees F or higher), nausea, vomiting, severe abdominal pain, septic shock, and ultimately death. A history of conjunctivitis 30 days prior to the onset of fever was also present in the documented BPF cases.

The physical presentation of children infected with BPF include purpuric skin lesions affecting mainly the face and extremities, cyanosis, rapid necrosis of soft tissue, particularly the hands, feet, nose, and ears. Analysis of the fatalities due to BPF showed hemorrhage in the skin, lungs, and adrenal glands. Histopathology showed hemorrhage, intravascular microthrombi and necrosis in the upper dermis, renal glomeruli, lungs, and hepatic sinusoids.

Individuals with "Klebsiella" pneumonia tend to cough up a characteristic sputum, as well as having fever, nausea, tachycardia and vomiting. "Klebsiella" pneumonia tends to affect people with underlying conditions, such as alcoholism.

"Mycoplasma" pneumonia (also known as "walking pneumonia" because it can spread bilaterally (“walk”) from one lung to the other) is a form of bacterial pneumonia caused by the bacterial species "Mycoplasma pneumoniae".

Pneumonia occurs when the lungs become infected, causing inflammation (swelling). Symptoms of pneumonia usually include:

- Fever (but older people may have lower than normal body temperature)

- Cough

- Shortness of breath

- Chills

- Sweating

- Chest pain that comes and goes with breathing

- Headache

- Muscle pain

- Excessive tiredness

- Nails may turn blue from lack of oxygen

Carbapenem-resistant Enterobacteriaceae (CRE) or carbapenemase-producing Enterobacteriaceae (CPE) are Gram-negative bacteria that are resistant to the carbapenem class of antibiotics, considered the drugs of last resort for such infections. They are resistant because they produce an enzyme called a carbapenemase that disables the drug molecule. The resistance can vary from moderate to severe. Enterobacteriaceae are common commensals and infectious agents. Experts fear CRE as the new "superbug". The bacteria can kill up to half of patients who get bloodstream infections. Tom Frieden, former head of the Centers for Disease Control and Prevention has referred to CRE as "nightmare bacteria". Types of CRE are sometimes known as KPC (Klebsiella pneumoniae carbapenemase) and NDM (New Delhi Metallo-beta-lactamase). KPC and NDM are enzymes that break down carbapenems and make them ineffective. Both of these enzymes, as well as the enzyme VIM (Verona Integron-Mediated Metallo-β-lactamase) have also been reported in Pseudomonas.

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.