Staphylococcus Lecture Notes PDF

Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 9 Microbiology Department of Dentisy ‫ـــــــــــــــــــ...

Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 9 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Staphylococcus Staphylococcus aureus causes abscesses, various pyogenic infections (e.g., endocarditis, septic arthritis, and osteomyelitis), food poisoning, scalded skin syndrome ,and toxic shock syndrome. It is one of the most common causes of hospital-acquired pneumonia, septicemia, and surgical-wound infections. It is an important cause of skin and soft tissue infections, such as folliculitis , cellulitis, and impetigo. It is the most common cause of bacterial conjunctivitis. Staphylococcus epidermidis causes prosthetic valve endocarditis and prosthetic joint infections. It is the most common cause of central nervous system shunt infections and an important cause of sepsis in newborns. Staphylococcus saprophyticus causes urinary tract infections, especially cystitis. Kawasaki syndrome is a disease of unknown etiology that may be caused by certain strains of S. aureus. Important Properties S. aureus has several important cell wall components and antigens: (1) Protein A is the major protein in the cell wall. It is an important virulence factor because it binds to the Fc portion of IgG at the complement-binding site, thereby preventing the activation of complement. As a consequence, no C3b is produced, and the opsonization and phagocytosis of the organisms are greatly reduced. Protein A is used in certain tests in the clinical laboratory because it binds to IgG and forms a “coagglutinate” with antigen– antibody complexes. The coagulase-negative staphylococci do not produce protein A. (2) Teichoic acids are polymers of ribitol phosphate. They mediate adherence of the staphylococci to mucosal cells. Lipoteichoic acids play a role in the induction of septic shock by inducing cytokines such as interleukin-1 (IL-1) and tumor necrosis factor (TNF) from macrophages. Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 9 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ (3) Polysaccharide capsule is also an important virulence factor. There are 11 serotypes based on the antigenicity of the capsular polysaccharide, but types 5 and 8 cause 85% of infections. Some strains of S. aureus are coated with a small amount of polysaccharide capsule, called a microcapsule. The capsule is poorly immunogenic, which has made producing an effective vaccine difficult. (4) Surface receptors for specific staphylococcal bacteriophages permit the “phage typing” of strains for epidemiologic purposes. Teichoic acids make up part of these receptors. (5) The peptidoglycan of S. aureus has endotoxin-like properties (i.e., it can stimulate macrophages to produce cytokines and can activate the complement and coagulation cascades). This explains the ability of S. aureus to cause the clinical findings of septic shock yet not possess endotoxin. Transmission Humans are the reservoir for staphylococci. The nose is the main site of colonization of S. aureus, and approximately 30% of people are colonized at any one time. People who are chronic carriers of S. aureus in their nose have an increased risk of skin infections caused by S. aureus. The skin, especially of hospital personnel and patients, is also a common site of S. aureus colonization. Hand contact is an important mode of transmission, and handwashing decreases transmission. Pathogenesis Staphylococcus aureus causes disease both by producing toxins and by inducing pyogenic inflammation. The typical lesion of S. aureus infection is an abscess. Abscesses undergo central necrosis and usually drain to the outside (e.g., furuncles and boils), but organisms may disseminate via the bloodstream as well. Foreign bodies, such as sutures and intravenous catheters, are important predisposing factors to infection by S. aureus. Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 9 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Type of Toxins The three clinically important exotoxins are enterotoxin, toxic shock syndrome toxin, and exfoliatin. (1) Enterotoxin causes food poisoning characterized by prominent vomiting and watery, nonbloody diarrhea. It acts as a superantigen within the gastrointestinal tract to stimulate the release of large amounts of IL-1 and IL-2 from macrophages and helper T cells, respectively. (2) Toxic shock syndrome toxin (TSST) causes toxic shock, especially in tampon-using menstruating women or in individuals with wound infections. Toxic shock also occurs in patients with nasal packing used to stop bleeding from the nose. (3) Exfoliatin causes “scalded skin” syndrome in young children. It is “epidermolytic” and acts as a protease that cleaves desmoglein in desmosomes, leading to the separation of the epidermis at the granular cell layer. (4) Several exotoxins can kill leukocytes (leukocidins) and cause necrosis of tissues in vivo. Of these, the two most important are alpha toxin and P-V leukocidin. Alpha toxin causes marked necrosis of the skin and hemolysis. P-V leukocidin is a pore-forming toxin that kills cells, especially white blood cells, by damaging cell membranes. (5) The enzymes include coagulase, fibrinolysin, hyaluronidase, proteases, nucleases, and lipases. Coagulase, by clotting plasma, serves to wall off the infected site, thereby retarding the migration of neutrophils into the site. Staphylokinase is a fibrinolysin that can lyse thrombi. Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 9 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Pyogenic Diseases (1) Skin and soft tissue infections are very common. These include abscess, impetigo furuncles, carbuncles, paronychia, cellulitis, folliculitis , hidradenitis suppurativa, conjunctivitis, eyelid infections (blepharitis and hordeolum), and postpartum breast infections (mastitis). (2) Septicemia (sepsis) can originate from any localized lesion, especially wound infection, or as a result of intravenous drug abuse. Sepsis caused by S. aureus has clinical features similar to those of sepsis caused by certain gram-negative bacteria, such as Neisseria meningitidis (3) Endocarditis may occur on normal or prosthetic heart valves, especially right-sided endocarditis (tricuspid valve) in intravenous drug users. (Prosthetic valve endocarditis is often caused by S. epidermidis.) (4) Osteomyelitis and septic arthritis may arise either by hematogenous spread from a distant infected focus or be introduced locally at a wound site. Staphylococcus aureus is a very common cause of these diseases, especially inchildren. Table Important Features of Pathogenesis by Staphylococci. Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 13 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Enterobacterceae Large group of Gram-negative rods Found worldwide in soil, water, and vegetation. Some are part of the normal intestinal flora of animals and humans Others are pathogenic for humans eg. salmonellae and shigellae, Involve many genera : Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia , Proteus, and many others Members of this family share these common characteristics: i. Gram-negative bacilli. ii. Aerobes or facultative anaerobes iii. All species ferment rather than oxidize glucose with the production of acid or acid and gas. iv. Either motile with peritrichous flagella or non-motile. v. Catalase positive (except for Shigella dysenteriae type 1 which is catalase- negative) vi. Oxidase-negative vii. Grow well on MacConkey agar 1 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 13 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Antigenic Structure Antigenic variability can be used for serogrouping the enteric bacteria Three major antigens involved : 1. O antigens (somatic antigens): polysaccharide chains in the lipopolysaccharide complex (LPS) of the outer membrane, Over 170 different O antigens have been described 2. H antigens (flagella antigens) consisting of protein located on flagella, So far 75 antigens have been identified. 3. K antigens (capsular): Linear polymers of the outer membrane consists of a repeated units of carbohydrate (sometimes proteins as well ) , More than 100 K antigens have been described The bacteria can be detected in agglutination assays with specific antibodies against these antigens< the antigenic formula of an E. coli can be O55:K5:H21. 2 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 13 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Main Virulence factors I. Surface antigens : 1. Somatic antigen (O Antigen), the function is Exerting endotoxic activity and also protects the bacteria from phagocytosis and the bactericidal effects of human complement system. 1. K Antigen: the function is Provide protection against phagocytosis and antibacterial activity of human normal serum 2. Fimbriae: Act as adhesion factor, Promote adhesion to human tissue such as colon, bladder, and others and Protect the bacteria from being washed away in the UT. II. Toxins 1-Endotoxins: The toxicity of the lipid A portion of LPS of outer membrane, the function is responsible for many of the systemic manifestations of infection. 2-Exotoxins : such as haemolysins, enterotoxins and cytotoxines, the function is usually lead to cause watery diarrhoea III. Antibiotic resistance factors (their encoding genes often are carried on plasmids and can be transferred between related species 3 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 13 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Medically important species E. coli, Klebsiella, Enterobacter, Proteus species Morganella, Providencia, Citrobacter, and Serratia They are members of the normal intestinal microbiota. Non-invasive They cause diseases only when they transfer into other body sites or tissues outside their normal intestinal sites or other less common intestinal sites When normal host defenses are inadequate, localized clinically important infections can result, and the bacteria may reach the bloodstream and cause sepsis. Body sites that can acquire clinically important infections: The urinary tract, biliary tract, and other sites in the abdominal cavity, any other body sites such as : bloodstream, meninges, prostate gland and lung 4 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Treponema pallidum Treponema pallidum is a gram-negative slender spiral shaped bacterium (spirochete) with regular coils and tapering ends, often growing in clusters. Rapid rotation about its axial filaments and endoflagella within periplasmic space at each end allow a corkscrew like motility. The helical structure of T. pallidum allows it to move in a corkscrew motion through the viscous media (mucus media). T. pallidum is micro aerophilic and an obligate internal parasite. Natural infection with T. pallidum is limited to the human host that cause's syphilis. Human infection is usually transmitted by sexual contact and the infectious lesion is on the skin or mucous membranes of genitalia. Acquired syphilis Is a chronic systemic venereal disease with multiple clinical presentations, entering the host via breaches or break in the skin or by penetrating the mucous membrane of genitalia. Spirochetes multiply locally at the site of entry, and some spread to nearby lymph nodes and then reach the bloodstream. Within 2-10 weeks 1 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ after infection, a papule develops to form an ulcer (“hard chancre”). This “primary lesion” always heals spontaneously. After 2-10 weeks, the “secondary” lesions appear. These consist of a red maculopapular rash anywhere on the body, including the hands and feet, and moist, pale papules (condylomas) in the anogenital region, axillae, and mouth. The patient may also have syphilitic meningitis, chorioretinitis, hepatitis, nephritis, or periostitis. The secondary lesions also subside spontaneously. Syphilitic infection may remain subclinical, and the patient may pass through the primary or secondary stage (or both) without symptoms or signs yet develop tertiary lesions. 2 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ In 30%, the untreated infection remains latent. In the remainder, the disease progresses to the “tertiary stage” characterized by the development of granulomatous lesions in the skin, bones, and liver; degenerative changes in the central nervous system (meningo vascular syphilis causing infected individuals to experience insomnia and changes in personality; or cardiovascular lesions. Another major complication of syphilis is its ability to increase the likelihood of transmission of HIV. In whole blood or plasma stored at 4℃, organisms remain viable for at least 24 hours, which is of potential importance in blood transfusions. Congenital Syphilis A pregnant woman with syphilis can transmit T. pallidum to the fetus through the placenta beginning in the 10th-15th weeks of gestation. Some of the infected fetuses die, and miscarriages result; others are stillborn at term. Others are born live but develop the signs of congenital syphilis in childhood, including interstitial keratitis, Hutchinson’s teeth, saddle nose, periostitis, and a variety of central nervous system anomalies. Adequate treatment of the mother during pregnancy prevents congenital syphilis Cutaneous disorders can be associated with a wide variety of dental manifestations that should be familiar to dermatologists. We sought to describe the development of the teeth, explain current dental terms, and review the dental manifestations of some dermatologic conditions. A MEDLINE search (1966-May 2007) was performed to find relevant articles pertaining to dental manifestations of dermatologic conditions. Dental manifestations are associated with a wide variety of skin diseases that include genetic, infectious, inflammatory, and immune disorders. 3 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬  Some Dental manifestations that causes by syphilis 1- Congenital erythropoietic , porphyria – 2-Congenital syphilis - Widely spaced upper incisors Red-colored teeth 3-Centrally notched upper incisors 4-Microdontia of molars (mulberry molars) (Hutchinson incisors) 5-Enamel hypoplasia of molars and incisors 4 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Diagnosis T. pallidum cannot be cultured in the lab and therefore cannot be investigated using conventional lab techniques because it cannot survive outside of mammalian cells. They do not stain well with aniline dyes. Clinical specimens include exudate or pus, tissue biopsy and serum. Tests for direct detection of T.pallidium -Animal inoculation (grown in the testicles of experimentally inoculated rabbit). -Dark field microscopy (morphology and movement) -Direct fluorescent antibody test (DFA-TP) (smear is stained with fluorescein- labeled anti- T.pallidium globulin and examined under fluorescent microscope) -Nucleic acid amplification test (PCR) 5 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Serological tests (antibody tests) 1-Non treponemal serologic tests: Non treponemal tests are called "non treponemal" because they detect antibodies that are not specifically directed against T. pallidum but may also be produced in several other conditions. The tests are highly sensitive but, false-positive results can be caused by other infections. A positive screening result must be confirmed with more specific treponemal tests. 2-VDRL (Venereal Disease Research Laboratory test): This test measures anti-lipid antibodies called reagin, which are formed by the host in response to lipids released from damaged cells early in infection with T.pallidum, and lipid-like material form the treponemal cell surface. A combination of regain and VDRL antigen form microscopic clumping called flocculation. 6 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ 3-RPR (Rapid Plasma Reagin test): The RPR antigen suspension is a carbon particle cardiolipin. The regain binds to the test antigen that cause flocculation. 4-Treponemal blood tests: These blood tests detect antibodies that specifically target T. pallidum. They are highly specific for syphilis, meaning other conditions are unlikely to cause a positive result. a) FTA-ABS (Fluorescent treponemal antibody absorption b) TP-PA (T. pallidum particle agglutination assay)--this test is sometimes performed instead of FTA-ABS because it is more specific. 7 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 14 + Lab 14 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Treatment Penicillin was the first known effective antibiotic for T. pallidum and remains the treatment of choice today. T. pallidum is also susceptible to amoxicillin and ceftriaxone, have been shown to be curative. Erythromycin, tetracycline and azithromycin are also able to inhibit T. pallidum but not as efficiently as the β-lactam antibiotics. Thus macrolides are relegated to second line antibiotic treatment status. Chloramphenicol, has been successful in treating syphilis due to concentrates in the CNS, it has utility in treating neurosyphilis. 8 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 15 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ THE HUMAN MICROFLORA The microbial colonization of all environmentally accessible surfaces of the body (both external and internal) begins at birth. Such surfaces are exposed to a wide range of microorganisms derived from the environment and from other persons. Each surface, however, because of its physical and biological properties, is suitable for colonization by only a proportion of these microbes. This results in the acquisition, selection and natural development of a diverse but characteristic microflora at distinct sites. Figure:Distribution of the resident human micro flora. The predominant groups of microorganism at some distinct anatomical sites are listed. 1 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 15 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ THE ORAL MICROFLORA IN HEALTH AND DISEASE The mouth is similar to other sites in the body in having a natural micro flora with a characteristic composition and existing, for the most part, in a harmonious relationship with the host. Perhaps more commonly than elsewhere in the body, this relationship can break down in the mouth and disease can occur. This is usually associated with: major changes to the biology of the mouth from exogenous sources (examples include: antibiotic treatment or the frequent intake of fermentable carbohydrates in the diet) or from endogenous changes such as alterations in the integrity of the host defenses following drug therapy, which perturb the natural stability of the micro flora. The presence of microorganisms at sites not normally accessible to them; for example, when oral bacteria enter the blood stream following tooth extraction or other traumas and are disseminated to distant organs, where they can cause abscesses or endocarditis. Mucosal surfaces The mouth is similar to other ecosystems in the digestive tract in having mucosal surfaces for microbial colonization. The microbial load is relatively low on such surfaces due to desquamation. However, the oral cavity does have specialized surfaces which contribute to the diversity of the micro flora at certain sites. The papillary structure of the dorsum of the tongue provides refuge for many microorganisms which would otherwise be removed by mastication and the flow of saliva. Such sites on the tongue can also have a low redox potential , which enable obligately anaerobic bacteria to grow. Indeed, the tongue can act as a reservoir for some of the Gram negative anaerobes that are implicated in the aetiology of periodontal diseases and are responsible for malodour. The mouth also contains keratinized (as in the palate) as well as non-keratinized stratified squamous epithelium which may influence the intra-oral distribution of some microorganisms. 2 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 15 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ The mouth The mouth is the only normally-accessible site in the body that has hard non- shedding surfaces for microbial colonization. Teeth do not appear in the mouth until after the first few months of life. The primary dentition is usually complete by the age of 3 years, and around 6 years the permanent teeth begin to erupt; this process is complete by about 12 years of age. Local ecological conditions will vary during these periods of change, which will in turn influence the composition of the resident microbial community at a site. Teeth (and dentures) allow the accumulation of large masses of microorganisms (predominantly bacteria) and their extracellular products, termed dental plaque. Plaque is an example of a biofilm and, while it is found naturally in health, it is also associated with dental caries and periodontal disease. In disease, there is a shift in the composition of the plaque microflora away from the 3 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 15 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ species that predominate in health.Each tooth is composed of four tissues – pulp, dentine, cementum and enamel. The pulp receives nerve signals and blood supplies from the tissues of the jaw via the roots. Thus the pulp is able to nourish the dentine and act as a sensory organ by detecting pain. Dentine makes up the bulk of the tooth and functions by supporting the enamel and protecting the pulp. Figure: Tooth structure in health and disease. 4 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 16 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ DISTRIBUTION OF THE RESIDENT ORAL MICROFLORA Lips and palate The lips form the border between the skin microflora (which consists predominantly of staphylococci, micrococci and Gram positive rods such as Corynebacterium and Propionibacterium spp.), and that of the mouth (which contains many Gram negative species and few of the organisms commonly found on the skin surface). Facultatively anaerobic streptococci comprise a large part of the microflora on the lips. Veillonella and Neisseria have also been found, but only in very low numbers (