See also Enterobacteriaceae 

Clinically relevant Gram-negative bacteria include Salmonella, Shigella, Escherichia, Klebsiella, Enterobacter, Seratia, Proteus, Morganella, Providencia, yersinia, Neisseria, Moraxella (Branhamella), the Acinetobacter and other less common genera. 

Gram-negative bacillis account for many nosocomial pneumonias, cinluding fatal ones. Gram negative bacillary pneumonias typically occur in infants, the elderly, or other immunocompormised hosts. The most important pathogen is Klebsiella pneumoniae, which causes Friedlander’s pneumonia. Other Gram negative pathogens associated with pneumonia include Pseudomonas aeruginosa, Escherichia coli, Enterobacter sp., Proteus sp., Serratia marcescens, and Acinetobacter.

Alcaligenes

Alcaligenes Faecalis: are gram-negative, rod shaped, motile, non-nitrate reducing, oxidase and catalase positive, alpha hemolytic, and citrate positive ogligate anaerobe commonly found in the environment such as the soil and water. On sheep blood agar the colonies are umbonate with an opaque center and a spreading edge. The pathogen is generally considered non-pathogenic but opportunistic infection does occur, often in the form of a urinary infection. 

Acinetobacteria

Acinetobacter is a group of bacteria (germs) commonly found in the environment, like in soil and water. While there are many types, the most common cause of infections is Acinetobacter baumannii, which accounts for most Acinetobacter infections in humans. See CDC

Acinetobacter baumannii can cause infections in the blood, urinary tract, and lungs (pneumonia), or in wounds in other parts of the body. It can also “colonize” or live in a patient without causing infections or symptoms, especially in respiratory secretions (sputum) or open wounds.

Bacteroidetes

Bacteroides fragilis is a prominent member of the gut microbiota within the phyla Bacteroidetes. This commensal bacterium produces unique capsular polysaccharides processed by antigen-presenting cells and activates CD4+ T cells to secrete inflammatory cytokines. Bacteroides fragilis is a gram-negative, anaerobic, rod-shaped bacterium. Bacteroides fragilis is a normal inhabitant of the human gastrointestinal tract, respiratory tract, and female urogenital tract. Bacteroides fragilis can cause opportunistic infections in humans, such as intra-abdominal abscesses and bacteremia.

Among the anaerobic pathogenic bacteria causing human infections, Bacteroides fragillis is the most fequent isolate, and multi-drug resistant strains are on the rise accounting for most treatment fialures. Metronidazone (MTZ) remains the antibiotic of choice for teh management of infections casued by anaerobes and resistance to MTZ is generally still low, however, resistant strains have been reported in regional surfvey studies. (Hoffman, “new funcitons of pirin proteins and a 2-ketoglutarate: Ferredoxin oxidoreductase ortholog in Bacterioids fragilis metabolism and their impact on antimicrobial susceptibility to metronidazole and amixicile” DOI: 10.1002, mbo3.1429 (July 2024). 

Bordetella

Bordetella are strictly aerobic, gram-negative nonmotile, slow growing, coccobacilli. One species, B. pertussis is the agent responsible for “whooping cough.” Virulence factors include –pertussis toxin (PT), –pertactin and filamentous hemagglutinin which are surface proteins for attachment to cells, and –the endotoxin LPS. Infants are at greatest risk (with 70% mortality rate). Because they are fastidious, culture can be difficult. Laboratory diagnosis can be by use of “cough plate” technique. Treatment includes erythromycin. There is a new vaccine which contains the PT and filamentous hemagglutinin toxoid.

Burkholderia

Burkholderia is a genus genus refers to a group of virtually ubiquitous Gram-negative, obligately aerobic, rod-shaped bacteria that are motile by means of single or multiple polar flagella, with the exception of Burkholderia mallei, which is nonmotile. The genus includes both animal and plant pathogens as well as some environmentally important species. For example, B. xenovorans is known for being catalase positive and its ability to degrade chloroganic pesticides and polychlorinated biphenyls. 

Due to their antibiotic resistance and the high motality rate from their associated diseases, B. mallei and B. pseudomallei are considered to be potential biological ware agents. 

Burkholderia pseudomallei: causes meioidosis, the third most common casue of death from infectious diseases in northeast Thailand. B. pseudomallei is commonly found in soil and water in southeast Asia and northeast Auralia. Melioidosis patietns were reported to be infected by inhalation or percutaneous inoculation from contaiminated muddy soil or stagnant water in endemic location, resulting in pneumonia and sepsis and upt to 40% of treated pateints died with this infecition. (Am J Trop Med Hyg. 2014, Mar 5; 90(3); 480-485). 

Burkholderia multivorans is a member of the Burkholderia cepacia complex, a group of water-related, Gram-negative opportunistic bacteria commonly found in soil and water. Outbreaks of these organisms have been associated with contaminated medications, aqueous medical products, and medical devices and are of clinical importance because these organisms can be antibiotic-resistant and can lead to severe infections, especially among immunocompromised and critically ill patients. During 2021–2022, contaminated ice and water from ice machines were linked to 23 Burkholderia multivorans cases at two southern California hospitals.  See CDC article

Chlamydia

Chlamydia are obligate intracellular bacteria (and require ATP from the host). Unlike other gram negative bacterial, they lack a  between the inner and outer membranes but do possess LPS. They are pleomorphic in that they take on different shapes such as rod like as well as coccus. Chlamydiae replicate via a unique growth cycle that is initatied by attachment of an infectious elementary body (EB) to microvilli of susceptible cells. Within 6-8 hours after entering the cell the EBs reorganize into metabolically active reticulate bodies (RB). The EB is rigid and resistant to disruption whereas the RB is fragile. 

Upon infection, chlamydiae have been found to interfere with cellular functions in various ways. One such way is the modulation of the host cell’s apoptosis machinery.

• C. trachomatis is the most frequent STD in the U.S. and a major cause of infertility in women, as well as the major cause of preventable blindness. Inoculation is through a break in the skin or membrane. Infections include PID in women as with N. gonorrhoeae, conjunctiva, and lymphogranuloma venereum (LGV) (male homosexuals at risk). C. trachomatis is a gram negative bacterium which causes which causes the STD Chlamydia. it is the most common STD which can be cured. Most people with the infection have no symptoms and do not seek testing. Chlamydia is most common among young people. Two-thirds of new chlamydial infections occur among youth aged 15-24 years. In 2021, chlamydia rates for African Americans/Blacks were six times that of Whites. Chlamydia is also common among MSM. Among MSM screened for rectal chlamydial infection, positivity ranges from 3.0% to 10.5%. Chlamydia spreads through vaginal, anal, or oral sex with someone with the infection. Semen does not have to be present to get or spread the infection. Some refer to chlamydia as a “silent” infection. This is because most people with the infection have no symptoms or abnormal physical exam findings. Studies find that the proportion of people with chlamydia who develop symptoms vary by setting and study methodology. 

  Anyone with the following genital symptoms should not have sex until they see a healthcare provider:

  • A discharge
  • A burning sensation when peeing
  • Unusual sores, or a rash

  Routine screening is not necessary for men. However, consider screening sexually active young men in clinical settings with a high prevalence of chlamydia. This can include adolescent clinics, correctional facilities, and STD clinics. Consider this when resources permit and do not hinder screening efforts in women. Screen sexually active MSM who have insertive intercourse for urethral chlamydial infection. Also screen MSM who have receptive anal intercourse for rectal infection at least yearly. Screening for pharyngeal infection is not recommended. MSM, including those with HIV, should receive more frequent chlamydia screening at 3- to 6-month intervals, if risk behaviors persist or if they or their sexual partners have multiple partners.To diagnose genital chlamydia in women using a NAAT, vaginal swabs are the optimal specimen. Urine is the specimen of choice for men. Urine is an effective alternative specimen type for women. See CDC

• C. pneumoniae causes a range of respiratory infections, more commonly upper airway infections but also pneumonia. Immunopathogenesis from inflammation is a hallmark of Chlamydia-induced disease. Chlamydial infections in vivo typically result in chronic inflammation characterized cellularly by the presence of activated monocytes and macrophages. At sites of chlamydial infections pro-inflammatory cytokines (IL-1?, IL-6, TNF?) and TH1 associated cytokines (IFNy and IL-12) have been identified. Promotion of any or all of these responses could be evoked by chronic infection as well as by chlamydial products such as LPS, heat shock proteins, and outer membrane proteins.

Chlamydia pneumonia has been linked to alzheimer’s disease.

• C psittaci causes psittacosis, an important and sometimes fatal respiratory disease usually cuased by close contact with infected birds.

Haemophilus

Haemophilus are gram negative bacilli that are parasites present on mucous membrane of humans and animal species. 

Haemophilus influenzae are small, gram negative, non spore forming, non-motile, aerobic anaerobes or facultative anaerobes. Virulence factors include –capsule (polyribitol phosphate (PRP), –pili which include adhesins, –LPS which leads to inflammation, –capsule and –outer membrane proteins IgA proteases.

Klebsiella pneumonia:

Klebsiella pneumonia is a gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterum.The colonies are mucoid, raised and shiny. 

Renckens Blood, 109(4), 2007) disclose that plasminogen activator inhibitor type-1 (PAI-1) overexpression in the lung markedly improved host defense against Klebsiella pneumonia and sepsis., demonstrating that PAI-1 is essential for host defense against severe Gram-negative pneumonia. 

Mycoplasma

Mycoplasma are a genus of bacteria and represent the smallest known cell with a diameter of about 10.1 microm (um). They lack a cell wall and as such, are unaffected by many common antiboiotics such as penicillin. Several species are pathogenic in humans, including M. genitalium, which is beleived to be involved in pelvic inflammatory diseases, and M. pneumoniae which is an important cause of atypical pneumonia and other respitory disorders. 

Mycoplasma penetrans: is pathogenic species infecting humans, typically be penetration into cells of the urogenital and respitatory tracts. 

Neisseria

Neisseria are aerobic, gram-negative cocci which occur in pairs. They are not motile nor do they have endospores. They are oxidase and catalase positive. 

Neisseria gonorrhoeae bacterium causes gonorrhea. N. gonorrhoeae infects the mucous membranes of the reproductive tract, including the cervix, uterus, and fallopian tubes in women, and the urethra in women and men. N. gonorrhoeae can also infect the mucous membranes of the mouth, throat, eyes, and rectum. Gonorrhea is a very common infectious disease. CDC estimates that approximately 1.6 million new gonococcal infections occurred in the United States in 2018, and more than half occur among young people aged 15-24. Gonorrhea is transmitted through sexual contact with the penis, vagina, mouth, or anus of an infected partner. Ejaculation does not have to occur for gonorrhea to be transmitted or acquired. Gonorrhea can also be spread perinatally from mother to baby during childbirth. Many men with gonorrhea are asymptomatic. When present, signs and symptoms of urethral infection in men include dysuria or a white, yellow, or green urethral discharge that usually appears one to fourteen days after infection. Most women with gonorrhea are asymptomatic. Untreated gonorrhea can cause serious and permanent health problems in both women and men. Urogenital gonorrhea can be diagnosed by testing urine, urethral (for men), or endocervical or vaginal (for women) specimens using nucleic acid amplification testing (NAAT). See CDC 

–Compared to other STDs: Bacterium Treponema Pallidum causes syphilis, and Neisseria Gonorrhoeae Bacterium causes gonorrhoea. Syphilis and gonorrhoea transmission can both be minimised through use of condomsm as they reduce skin-to-skin contact and help to stop bodily fluids from the anus, mouth, penis or front hole from being shared. Syphilis and gonorrhoea are both transmitted by sex; however, their modes of transmission are different. Syphilis transmission most commonly occurs by skin-to-skin contact with sores (known as “chancres”), usually found on your genitals, arse, mouth, lips or skin. Gonorrhoea is primarily transmitted during sex by infected fluids such as semen, precum and front hole fluids. See ACON Chlamydia is more common than gonorrhea. According to a 2017 report, over 1.7 million cases of chlamydia were reported in the United States, while just over 550,000 cases of gonorrhea were documented. With chlamydia, symptoms may not appear for a few weeks after you’ve contracted the infection. And with gonorrhea, people who have female anatomy may never experience any symptoms at all or may only show mild symptoms, while people who have male anatomy are more likely to have symptoms that are more severe. See Healthline

–Treatment: Neisseria gonorrhea is showing resistance to mutliple classes of antibiotics. It exhigits significant variation in the antigens on its surface proteins and can easil alter its surface structure during infection. Limma Tech Biologics is developing a vaccine against N. gonorrhea that is in preclinical testing. To target the issue of surface variability, the candidate contains multiple immunogenic antigens that are invariable or conserved. 

Neisseria meningtidis: causes meningococcal disease, which is a rare but life-threatning illness that reqire prompot antibiotic treatment as wll as antibiotic prophylaxis for close contacts.

–Transmission/Prevention: N. meningitidis is transmitted person-to-person via respiratory droplets and oropharyngeal secretions. Asymptomatic persons can carry N. meningitids and transmit the bacteria to otehrs, potentially cuasing illness among susceptible persons. Outbreaks can occur in conjunciton with large gathering. (McNamara, “Cases of Meningococcal disease assocaited with travel to Saudi Arabia for Umrah Pilgrimage–United States, United Kingdom, and France, 2024” Mobidity and Mortality Weekly Report, vol 73, No. 22, 2024).  

Large meningococcal disease outbreaks assocaited with Hajj and Umrah were reproted in 1987, 1992, and 2000-2001. Since 2002, Saudi Arabia has required documentation of either a quadrivalent eningococcal 9MenACWY) polysaccharide vaccine within the alst 3 years or a MenACWY conjugate vacine within the last 5 years and adminsitered greater or equal to 10 days before arriveal for all pilgrims age 1 year or mroe enteirng the country. 

Prophylaxis with rifampin, ceftriaxone, or azithromycin should be preferentially considered instead of ciprofloxacin for close contacts of pateints with Saudi Arabia travel assocaited cases. McNamara, “Cases of Meningococcal disease assocaited with travel to Saudi Arabia for Umrah Pilgrimage–United States, United Kingdom, and France, 2024” Mobidity and Mortality Weekly Report, vol 73, No. 22, 2024).

–Treatment:

First line options for prophylaxis are ifampin, ciprofloxacin and ceftriaxone; azithromycin can also be used in areas with ciprofloxacin-resistant strains. Historically, antibotic resistance in N. meningitidis has been uncommon in the U.S. However, in 2020, CDC idnetified 100 ciprofloxacin and penicillin resistant N. meningitidis serogroup Y (Nmy) isoaltes form cases occurring in 2019 and 2020. (CDC, “Selection of antibiotics as prophylaxis for clsoe contacts of patients with Meningococcal Disease in areas with ciproloxacin resistance -United States, 2024”. 

Proteus

Proteus vulgaris: are rod-shaped which inhabit the intestinal tracts of humans and animals. They are opportunistic and known to cause urinary track infection and wound infections. It ferments glucose and amygdalin, but does not ferment mannitol or lactose. 

Pseudomonas

Pseudomonas aeruginosa are gram – rods, aerobes (do not ferment glucose), Lactose negative (so will not form pinkish red colonies on a MacConkey Agar as will Lac+ bacteria), oxidase positive opportunistic pathogens (cause chronic colonization in cystic fibrosis patients) which may be pignmented. They are very stable organisms that are seen around hospitals and tend to get into catheters, IV lines, etc. In healthy people they tend to be limited to puncture wounds, swimmer’s ear and contact lens infections. Virulence factors include –endotoxins, –exotoxin A and –some may have a capsule (alginate).

Infections with P. aeruginosa are critical in ventilated and poly-traumatized patients and these bacteria cause frequent and chronic pulmonary infections in patients with cystic fibrosis. Kirschnek discloses that annexin II is a novel receptor for P. aeruginosa and that incubation of the bacteria with recobminant, soluble annexin II prevents internalization of P. aeurginosa into human epithelial cells. Typically, individuals infected with STEC develop abdominal pain and mostly bloody diarrhea (hemorrhagic colitis) within 2-5 days, following exposure. Althought the illness sually resoves without sequelae, hemolytic uremic syndrome (HUS) can occur several days following the onset of bloody diarrhea in 5-10 % of susceptible individuals, particulalry children and the elderly. HUS, characterized by hemolytic anemial, thrombocytopenia, acute renal damage, and various degress of central nervous system complications, can result in death or chronic, irreversible renal dysfunction. STEC produce one or two genetically and antigenically distinct exotoxins designated Shiga toxin 1 (Stx1) an Stx2, of which Stx2 is the primary virulence factor for HUS.  (Biochemical and Biophysical Reserch Communications, 327 (2005) 900-906).  

See CDC

Rickettsiae

The Rickettsiae consists of aerobic, gram-negative bacilli that are obligate intracellular parasites. Treatment of the diseases below is with tetracycline.

Rickettsia rickettsii are the agent of Rocky Mountain spotted fever (RMSF). They are found in the cytoplasm in triad. Ticks are their vector and rodents, dogs and ticks are their reservoir. Detection if with direct IFA, PCR or retrospective serology. Treatment is with tetracyclines.

Coxiella burnettii causes Q fever. A common symptom is chronic endocarditis. The reservoir are animals but transmission is via inhalation rather than vectors like ticks. The organisms is environmentally resistant and can survive in soil for years. It is found inside cell vacuoles rather than the cytoplasms as with rickettsii.

Ehrlichia cahffeensis is the agent of erhlichiosis which is similar to RMSF but without a rash. Treatment is with tetracylcine.

Rickettsia typhi is a small, aerobic, obligate intracellular, rod shaped gram negative bacterium. It belongs to the typhus group of the Rickettsia genus, along with R. prowazekii. 

–Fleaborne typhus (also known as murine typhus), a widely distributed vectorborne zoonosis caused by Rickettsia typhi, is a moderately severe, but infrequently fatal illness; among patients who receive doxycycline, the case-fatality rate is <1%. Fleaborne typhus is transmitted from infected fleas by inoculation of flea feces into the flea bite site, a skin abrasion, or mucous membranes. (Alarcon)

Salmonella

Salmonella are a genus of rod shaped non-spore forming bacteria.

Salmonella tryphimurium: are predominantly found in the intestinal lumen which cause gasteroenteritis in humans and other animals. When the bacteria cells enter epithelial cells lining the intestine, they cause damage to the microvilli on the surface of the cell. This causes a rush of white blood cells into the mucosa which upsets the ratios between absorption and secretion and leads to diarrhea. 

Shigella

Shigella is a genus of gram negative rod shaped facultative anaerobic nonspore forming bacteria. Shigella causes bacillary dysentery which is a serious illness characterized by abdominal cramps, nausea, fever and a bloody and mucus diarrhea. Shigella are classified into four species based on their antigen characteristics. Shigella species are related to E. coli. In fact, enteroinvasive E. coli are strains that have the ability to cause dysentery using the same method as Shigella. Despite their relatedness, Shigella could always be spearated form E. coli based on their physiological and biochemical characteristics. Most of the E. coli strains are motile, lysine decarboxylase (LDC) positive,, form gas from D-glucose and are indol positive. In contrast, Shigella strains are always non-motile, always LDC negative and never form gas from D-glucose, except Shigella flexneri serotype 6, which is indol negative. 

Shiga first described Shigella as Bacillus dysenteriae in 1898. He named it Bacillus because it seemed to be related to Vacillus coli, which is now called E. coli. In the 1940s, Ewing proposed to classify four species in the new genus, Shigella, namely S. dysenteriae, S. flexneri, S. boydii and S. sonnei, based on the antigen characteristics of those species.

Shiga toxin (Stx) producing E. coli (particularly E. coli O157:H7): 

Shiga toxin producing Escherichia coli (STEC) 0157.H7 is an enteric illness that can cause hemolytic uremic syndrom (HUS), a severe, life threatening condition which affects the kidneys; young children (ages <5) are among the most susceptible to HUS. (CDC, “Shiga Toxin-Producing Excherichia coli 0157:H7 illness outbreak associated with untreated, pressurized, municipal irrigation water -Utah, 2023” (May 9, 2024).

Shiga toxin producing E coli have occurred with increasing fequency. Within the US alone, it is estimated that annually there are about 100k of STEC infection, of which some 73k are due to infection with O157:H7. The most common sources of infction are food and water contaminated with animal and human effluents, of which cattle are considered the primary animal reservoir. Infection via contacted with infected individuals, however, is also important. (Tzipori, Clinical Microbiology Reviews, Oct. 2004, p. 926-941).

Durng July-september 2023, an outbreak of Shiga toxin producing Escherichia coli 0157:H7 illness among children in city A Utah, casued 13 confirmed illnesses, 7 pateints were hospitalized, including two with hemolytic uremic syndrome. Pulbic health officials linked the illnesses to utnreated, pressureized, municpal irrigation water. 12 of 13 ill chidren reported playing in or drinking this water. Microbial source tracking identified birds and ruminants as potential sources of fecal contamination of the water. Investigators submitted samples of bird feces to the Utah Public Health Laboratory for STEC 0157.H7 culture and submitted all other environmental sampels to CDC for cultrue of sTEC 0157:H7, followed by WGS of confirmed STEC 0157:H7 isolates. (CDC, “Shiga Toxin-Producing Excherichia coli 0157:H7 illness outbreak associated with untreated, pressurized, municipal irrigation water -Utah, 2023” (May 9, 2024). 

Gram – Bacilli

• N. meningitides are gram-negative diplococci and are aerobic or facultative anaerobes. They are both catalase and oxidase positive. Virulence factors include –encapsulation, –hemolysis which lyse RBCs, –toxins, –pili which allows N. meningitidis to colonize the nasopharynx. N. meningitidis is subdividied into serogroups (A,B,C,Y and W135). The nasopharyngeal rates are highest in school-age children. Meningococcus bacteria are spread through the exchange of respiratory and throat secretions like spit (e.g., by living in close quarters, kissing). Meningococcal disease can be treated with antibiotics, but quick medical attention is extremely important. Keeping up to date with recommended vaccines is the best defense against meningococcal disease. There are vaccines to each of these serogroups. See CDC for more information

• N. gonorrhoeae are diplococci which cause pelvic inflammatory disease in women and can cause infertility if left untreated. They are easily transmitted from person to person and is the second most commonly reported STD in the US. It is found only in humans and its reservoir is asymptomatically infected women. Their virulence factors include — a capsule which helps them evade phagocytosis (unlike N. meningitidis), –pili composed of pillins that have antigen variation (facilitates lack of immunity) and –additional proteins in the outer membrane which facilitate attachment to mucosal cells. 

• Shigella flexneri: can cause diarrhea in humans which can typically be treated with antibiotics although some strains have become resistant. 

• S. marcescens: is a rod-shapped gram negative bacteria which is a facultative anaerobe. It is an opportunistic bacteria and commonly involved in hospital acquired infections, particularly catheter assocaited bacteremia., urinary tract infections, and wound infections. It is responsible for about 1.4% of hosptial aquired infections in the US. It is also abundant in the environment and prefers damp conditions. For example, it is commonly found growing in bathrooms, especially on tile grout, shower curtains, toilet water lines, and basins. It is pink or pink to orange. It feeds off phopsphorous containing items or fatty substances such as soap and shampoo residue. It can also be found in dirt. and subgingiviral biofilm of teeth. It produces a a reddish-organe trippyrole pigment. Rinsing and drying surfaces after use can prevent its establishment by removing its food surfaces and making its environment less hospital. Complete erradication of the bacterial is difficult but can be accomplished with by a bleach based disinfectant. 

Other Zoonotic Diseases

Zoonoses is the transmission of disease from animals to human. Bacteria which cause zoonotic disease are often highly virulent and are difficult to culture or diagnose (cause “fevers of unkown origin” FOU). 

• Francisella tularensis is the causative agent of Tularemia otherwise known as “rabbit, tick or deerfly fever”. It is a small gram negative coccobacillus, strictly aerobe (nonfermenter), facultative intracellular pathogen. Collection and processing of specimens for its isolation is extremely hazardous because it can penetrate through the skin and mucous membrane due to its small size or be inhaled if aerosols are produced. 

• Brucella genus consists of several species including B. abortus. They are gram negative, aerobic coccobacillus. They are nonmotile, non capsulated facultative intracellular pathogens (can survive in macrophages). Symptoms include an “undulant” or intermittent fevor. Treatment is by way of tetracylcine. 

• Bartonella are gram negative rods obligate intercellular bacteria (rickettsia-like). They include several species including B henselae which causes “cat-scratch disease” as well as bacillary angiomatosis (BA) which affects immuncompromised patients (like those with AIDS and causes lesions similar to Kaposi’s sarcoma). They respond to antibiotics. 

Gram – Spirochetes

Spirochetes have a gram negative cell envelope but no LPS. They are typically thin and not visible with gram stain. The first one below Treponema pallidum is an STD whereas the other two are zoonotic.

Treponema pallidum

1. Treponema pallidum is the agent for syphilis. It is a thin and long spirochete that can not be seen using standard microscopic techniques. Instead, diagnosis is by darkfield microscopy or serology. It is sensitive to penicillin but more difficult to treat in patients with HIV infections. (See Syphillis outline)

Borrelia Burgdorferi

Borrelia burgdorferi is the agent for lyme disease. They are larger than other spirochetes. Reservoirs for these bacteria include rodents, deer, etc. Vectors are ticks. 

Leptospira interrogans

Leptospira interrogans are aerobic, motile and can be cultivated in the laboratory. Reservoirs include dogs, cattle, rodents, etc (making control difficult). They are transmitted by exposure to contaminated water or infected animals and include a flu-like illness. Treatment is with penicillin or tetracycline.