In general, the Duke criteria should be fulfilled in order to establish the diagnosis of endocarditis. The blood tests C reactive protein (CRP) and procalcitonin have not been found to be particularly useful in helping make or rule out the diagnosis.

As the Duke criteria rely heavily on the results of echocardiography, research has addressed when to order an echocardiogram by using signs and symptoms to predict occult endocarditis among patients with intravenous drug abuse and among non drug-abusing patients. Unfortunately, this research is over 20 years old and it is possible that changes in the epidemiology of endocarditis and bacteria such as staphylococci make the following estimates incorrect.

The transthoracic echocardiogram has a sensitivity and specificity of approximately 65% and 95% if the echocardiographer believes there is 'probable' or 'almost certain' evidence of endocarditis.

Bacteremia is most commonly diagnosed by blood culture, in which a sample of blood drawn from the vein by needle puncture is allowed to incubate with a medium that promotes bacterial growth. If bacteria are present in the bloodstream at the time the sample is obtained, the bacteria will multiply and can thereby be detected.

Any bacteria that incidentally find their way to the culture medium will also multiply. For example, if the skin is not adequately cleaned before needle puncture, contamination of the blood sample with normal bacteria that live on the surface of the skin can occur. For this reason, blood cultures must be drawn with great attention to sterile process. The presence of certain bacteria in the blood culture, such as S"taphylococcus aureus", "Streptococcus pneumoniae", and "Escherichia coli" almost never represent a contamination of the sample. On the other hand, contamination may be more highly suspected if organisms like "Staphylococcus epidermidis" or "Propionibacterium acnes" grow in the blood culture.

Two blood cultures drawn from separate sites of the body are often sufficient to diagnose bacteremia. Two out of two cultures growing the same type of bacteria usually represents a real bacteremia, particularly if the organism that grows is not a common contaminant. One out of two positive cultures will usually prompt a repeat set of blood cultures to be drawn to confirm whether a contaminant or a real bacteremia is present. The patient's skin is typically cleaned with an alcohol-based product prior to drawing blood to prevent contamination. Blood cultures may be repeated at intervals to determine if persistent — rather than transient — bacteremia is present.

Prior to drawing blood cultures, a thorough patient history should be taken with particular regard to presence of both fevers and chills, other focal signs of infection such as in the skin or soft tissue, a state of immunosuppression, or any recent invasive procedures.

Ultrasound of the heart is recommended in all those with bacteremia due to "Staphylococcus aureus" to rule out infectious endocarditis.

The CDC states that PCR testing from a single blood draw is not sufficiently sensitive for "B." "henselae" testing, and can result in high false negative rates due to a small sample volume and levels below the limit of molecular detection.

"Bartonella" spp. are fastidious, slow-growing bacteria that are difficult to grow using traditional solid agar plate culture methods due to complex nutritional requirements and potentially a low number of circulating bacteria. This conventional method of culturing "Bartonella" spp. from blood inoculates plated directly onto solid agar plates requires an extended incubation period of 21 days due to the slow growth rate.

"Bartonella" growth rates improve when cultured in an enrichment inoculation step in a liquid insect-based medium such as "Bartonella" α-Proteobacteria Growth Medium (BAPGM) or Schneider’s Drosophila-based insect powder medium. Several studies have optimized the growing conditions of "Bartonella" spp. cultures in these liquid media, with no change in bacterial protein expressions or host interactions "in vitro". Insect-based liquid media supports the growth and co-culturing of at least seven "Bartonella" species, reduces bacterial culturing time and facilitates PCR detection and isolation of "Bartonella" spp. from animal and patient samples. Research shows that DNA may be detected following direct extraction from blood samples and become negative following enrichment culture, thus PCR is recommended after direct sample extraction and also following incubation in enrichment culture. Several studies have successfully optimized sensitivity and specificity by using PCR amplification (pre-enrichment PCR) and enrichment culturing of blood draw samples, followed by PCR (post-enrichment PCR) and DNA sequence identification.

Diagnosis of subacute bacterial endocarditis can be done by collecting three blood culture specimens over a 24-hour period for analysis, also it can usually be indicated by the existence of:

- Osler's nodes

- Roth's spots

- Nail clubbing

The standard treatment is with a minimum of four weeks of high-dose intravenous penicillin with an aminoglycoside such as gentamicin.

The use of high-dose antibiotics is largely based upon animal models.

Leo Loewe of Brooklyn Jewish Hospital was the first to successfully treat subacute bacterial endocarditis with penicillin. Loewe reported at the time seven cases of subacute bacterial endocarditis in 1944.

The presence of bacteria in the blood almost always requires treatment with antibiotics. This is because there are high mortality rates from progression to sepsis if antibiotics are delayed.

The treatment of bacteremia should begin with empiric antibiotic coverage. Any patient presenting with signs or symptoms of bacteremia or a positive blood culture should be started on intravenous antibiotics. The choice of antibiotic is determined by the most likely source of infection and by the characteristic organisms that typically cause that infection. Other important considerations include the patient's past history of antibiotic use, the severity of the presenting symptoms, and any allergies to antibiotics. Empiric antibiotics should be narrowed, preferably to a single antibiotic, once the blood culture returns with a particular bacteria that has been isolated.

Initial diagnosis may be via symptoms, but is usually confirmed via an antigen and antibody test. A PCR-based test is also available. Although any of these tests can confirm psittacosis, false negatives are possible and so a combination of clinical and lab tests is recommended before giving the bird a clean bill of health. It may die within three weeks.

The organism should be cultured and antibiotic sensitivity should be determined before treatment is started. Amoxycillin is usually effective in treating streptococcal infections.

Biosecurity protocols and good hygiene are important in preventing the disease.

Vaccination is available against "S. gallolyticus" and can also protect pigeons.

Blood analysis shows leukopenia, thrombocytopenia and moderately elevated liver enzymes. Differential diagnosis must be made with typhus, typhoid and atypical pneumonia by Mycoplasma, Legionella or Q fever. Exposure history is paramount to diagnosis.

Diagnosis involves microbiological cultures from respiratory secretions of patients or serologically with a fourfold or greater increase in antibody titers against "C. psittaci" in blood samples combined with the probable course of the disease. Typical inclusions called "Leventhal-Cole-Lillie bodies" can be seen within macrophages in BAL (bronchoalveolar lavage) fluid. Culture of "C. psittaci" is hazardous and should only be carried out in biosafety laboratories.

Post-mortem findings include friable internal organs, abdominal effusion and evidence of sepsis in the joints, heart valves and brain.

Bacteria can usually be cultured from tissues collected at necropsy or identified by microscope examination.

As the infection is usually transmitted into humans through animal bites, antibiotics usually treat the infection, but medical attention should be sought if the wound is severely swelling. Pasteurellosis is usually treated with high-dose penicillin if severe. Either tetracycline or chloramphenicol provides an alternative in beta-lactam-intolerant patients. However, it is most important to treat the wound.

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

Cat-scratch disease is characterized by granulomatous inflammation on histological examination of the lymph nodes. Under the microscope, the skin lesion demonstrates a circumscribed focus of necrosis, surround by histiocytes, often accompanied by multinucleated giant cells, lymphocytes, and eosinophils. The regional lymph nodes demonstrate follicular hyperplasia with central stellate necrosis with neutrophils, surrounded by palisading histiocytes (suppurative granulomas) and sinuses packed with monocytoid B cells, usually without perifollicular and intrafollicular epithelioid cells. This pattern, although typical, is only present in a minority of cases.

The Warthin–Starry stain can be helpful to show the presence of "B. henselae", but is often difficult to interpret. "B. henselae" is difficult to culture and can take 2–6 weeks to incubate. The best diagnostic method currently available is polymerase chain reaction, which has a sensitivity of 43-76% and a specificity (in one study) of 100%.

Definite diagnosis of brucellosis requires the isolation of the organism from the blood, body fluids, or tissues, but serological methods may be the only tests available in many settings. Positive blood culture yield ranges between 40% and 70% and is less commonly positive for "B. abortus" than "B. melitensis" or "B. suis". Identification of specific antibodies against bacterial lipopolysaccharide and other antigens can be detected by the standard agglutination test (SAT), rose Bengal, 2-mercaptoethanol (2-ME), antihuman globulin (Coombs’) and indirect enzymelinked immunosorbent assay (ELISA). SAT is the most commonly used serology in endemic areas. An agglutination titre greater than 1:160 is considered significant in nonendemic areas and greater than 1:320 in endemic areas. Due to the similarity of the O polysaccharide of "Brucella" to that of various other Gram-negative bacteria (e.g. "Francisella tularensis", "Escherichia coli", "Salmonella urbana", "Yersinia enterocolitica", "Vibrio cholerae", and "Stenotrophomonas maltophilia") the appearance of cross-reactions of class M immunoglobulins may occur. The inability to diagnose "B. canis" by SAT due to lack of cross-reaction is another drawback. False-negative SAT may be caused by the presence of blocking antibodies (the prozone phenomenon) in the α2-globulin (IgA) and in the α-globulin (IgG) fractions. Dipstick assays are new and promising, based on the binding of "Brucella" IgM antibodies, and found to be simple, accurate, and rapid. ELISA typically uses cytoplasmic proteins as antigens. It measures IgM, IgG, and IgA with better sensitivity and specificity than the SAT in most recent comparative studies. The commercial Brucellacapt test, a single-step immunocapture assay for the detection of total anti-"Brucella" antibodies, is an increasingly used adjunctive test when resources permit. PCR is fast and should be specific. Many varieties of PCR have been developed (e.g. nested PCR, realtime PCR and PCR-ELISA) and found to have superior specificity and sensitivity in detecting both primary infection and relapse after treatment. Unfortunately, these have yet to be standardized for routine use, and some centres have reported persistent PCR positivity after clinically successful treatment, fuelling the controversy about the existence of prolonged chronic brucellosis. Other laboratory findings include normal peripheral white cell count, and occasional leucopenia with relative lymphocytosis. The serum biochemical profiles are commonly normal.

Due to the non-invasive nature of NBTE, clinical examination may or may not reveal a new murmur.

An embolic stroke may be the first feature to suggest the diagnosis of NBTE. An echocardiograph may be used to further assess for valvular lesions.

Infective endocarditis is an infection of the inner surface of the heart, usually the valves. Symptoms may include fever, small areas of bleeding into the skin, heart murmur, feeling tired, and low red blood cells. Complications may include valvular insufficiency, heart failure, stroke, and kidney failure.

The cause is typically a bacterial infection and less commonly a fungal infection. Risk factors include valvular heart disease including rheumatic disease, congenital heart disease, artificial valves, hemodialysis, intravenous drug use, and electronic pacemakers. The bacterial most commonly involved are streptococci or staphylococci. Diagnosis is suspected based on symptoms and supported by blood cultures or ultrasound.

The usefulness of antibiotics following dental procedures for prevention is unclear. Some recommend them in those at high risk. Treatment is generally with intravenous antibiotics. The choice of antibiotics is based on the blood cultures. Occasionally heart surgery is required.

The number of people affected is about 5 per 100,000 per year. Rates, however, vary between regions of the world. Males are affected more often than females. The risk of death among those infected is about 25%. Without treatment it is almost universally fatal.

Diagnosis of nocardiosis can be done by a doctor using various techniques. These techniques include, but are not limited to: a chest x-rays of the lung, a bronchoscopy, a brain/lung/skin biopsy, or a sputum culture.

However, diagnosis may be difficult. Nocardiae are gram positive weakly acid-fast branching rod-shaped bacteria and can be visualized by a modified Ziehl–Neelsen stain like Fite-Faraco method. In the clinical laboratory, routine cultures may be held for insufficient time to grow nocardiae, and referral to a reference laboratory may be needed for species identification. Infiltration and pleural effusion are usually seen via x-ray.

This condition is diagnosed by detecting the bacteria in skin, blood, joint fluid, or lymph nodes. Blood antibody tests may also be used. To get a proper diagnosis for rat-bite fever, different tests are run depending on the symptoms being experienced.

To diagnosis streptobacillary rat-bite fever, blood or joint fluid is extracted and the organisms living in it are cultured. Diagnosis for spirillary rat bite fever is by direct visualization or culture of spirilla from blood smears or tissue from lesions or lymph nodes. Treatment with antibiotics is the same for both types of infection. The condition responds to penicillin, and where allergies to it occur, erythromycin or tetracyclines are used.

Common situations in which antibiotics are overused include the following:

- Apparent viral respiratory illness in children should not be treated with antibiotics. If there is a diagnosis of bacterial infection, then antibiotics may be used.

- When children with ear tubes get ear infections, they should have antibiotic eardrops put into their ears to go to the infection rather than having oral antibiotics which are more likely to have unwanted side effects.

- Swimmer's ear should be treated with antibiotic eardrops, not oral antibiotics.

- Sinusitis should not be treated with antibiotics because it is usually caused by a virus, and even when it is caused by a bacteria, antibiotics are not indicated except in atypical circumstances as it usually resolves without treatment.

- Viral conjunctivitis should not be treated with antibiotics. Antibiotics should only be used with confirmation that a patient has bacterial conjunctivitis.

- Older persons often have bacteria in their urine which is detected in routine urine tests, but unless the person has the symptoms of a urinary tract infection, antibiotics should not be used in response.

- Eczema should not be treated with oral antibiotics. Dry skin can be treated with lotions or other symptom treatments.

- The use of topical antibiotics to treat surgical wounds does not reduce infection rates in comparison with non-antibiotic ointment or no ointment at all.

Diagnosis is usually based on serology (looking for an antibody response) rather than looking for the organism itself. Serology allows the detection of chronic infection by the appearance of high levels of the antibody against the virulent form of the bacterium. Molecular detection of bacterial DNA is increasingly used. Culture is technically difficult and not routinely available in most microbiology laboratories.

Q fever can cause endocarditis (infection of the heart valves) which may require transoesophageal echocardiography to diagnose. Q fever hepatitis manifests as an elevation of alanine transaminase and aspartate transaminase, but a definitive diagnosis is only possible on liver biopsy, which shows the characteristic fibrin ring granulomas.

The prognosis of nocardiosis is highly variable. The state of the host's health, site, duration, and severity of the infection all play parts in determining the prognosis. As of now, skin and soft tissue infections have a 100% cure rate, and pleuropulmonary infections have a 90% cure rate with appropriate therapy. The cure rate falls to 63% with those infected with dissemented nocardiosis, with only half of those surviving infections that cause brain abscess. Additionally, 44% of people who are infected in the spinal cord/brain die, increasing to 85% if that person has an already weakened immune system. Unfortunately, there is not a preventative to nocardiosis. The only recommendation is to protect open wounds to limit access.

Though antibiotics are required to treat severe bacterial infections, misuse has contributed to a rise in bacterial resistance. The overuse of fluoroquinolone and other antibiotics fuels antibiotic resistance in bacteria, which can inhibit the treatment of antibiotic-resistant infections. Their excessive use in children with otitis media has given rise to a breed of bacteria resistant to antibiotics entirely.

Widespread use of fluoroquinolones as a first-line antibiotic has led to decreased antibiotic sensitivity, with negative implications for serious bacterial infections such as those associated with cystic fibrosis, where quinolones are among the few viable antibiotics.