Introduction
Intestinal bacteria play an important role in the nutritional, physiological, immunological, and protective functions of the host (Vispo and Karasov, Reference VISPO and KARASOV1997) and can be influenced by the diet (Rehman et al., Reference REHMAN, VAHJEN, AWAD and ZENTEK2007). Research into possible alternatives for in-feed antibiotics has focused on the activities of potentially beneficial bacteria. The most promising approach is the addition of specific ingredients to the diets that can enhance the ability of the indigenous intestinal bacteria to protect the host against pathogenic infections, an effect known as ‘colonisation resistance’. Changes in the dietary distribution or nutrient densities may have dramatic effects on the intestinal bacterial community (Gibson et al., Reference GIBSON, WILLEMS, READING and COLLINS1996; Reid and Hillman, Reference REID and HILLMAN1999) that can influence the host's ability to digest and absorb dietary nutrients. For this purpose, fermentable carbohydrates are considered to have potentially beneficial effects on the composition and activity of the intestinal bacteria (Gibson and Roberfroid, Reference GIBSON and ROBERFROID1995). Prebiotics are non-digestible food or feed ingredients that beneficially affect the host by selectively stimulating the growth, activity or both, of one or a limited number of bacterial species in the intestine of the host (Gibson et al., Reference GIBSON, PROBERT, VAN LOO, RASTALL and ROBERFROID2004). For a substrate to be a candidate as prebiotic, it:
1. must be neither hydrolyzed, nor absorbed in the upper part of the gastrointestinal tract,
2. must be selectively fermented by one or a limited number of potentially beneficial bacteria which are stimulated to grow and/or be metabolically activated,
3. must influence the composition of the hind gut microbiota towards a healthier composition,
4. must preferably induce effects that are beneficial to the host health. (Gibson and Roberfroid, Reference GIBSON and ROBERFROID1995).
Normally, prebiotics consist of a carbohydrate fraction (oligosaccharides, non-starch polysaccharides and starch). Oligosaccharides are believed to stimulate the growth of certain bacteria, especially Lactobacillus spp. and Bifidobacterium spp. The inhibition of pathogenic bacteria like Salmonella spp., Campylobacter spp. or putrefactive bacteria like Clostridium perfringens by the oligosaccharides can in part be explained by their fermentation products in the intestine (Gibson and Wang, Reference GIBSON and WANG1994). However, the acid production from these bacteria and an acidic pH are not the sole mechanisms of inhibition, as other mechanisms are also involved which are yet to be elucidated. For instance, some of lactic acid bacteria also produce antibacterial substances, like bacteriocine, that inhibit the growth of different enteropathogens (Lasagno et al., Reference LASAGNO, BEOLETTO, SESMA, RAYA, FONT DEVALDEZ and ERASO2002).
The impact of prebiotics on the intestinal bacteria is still relatively poorly understood in broilers. This paper describes the potential impact of different prebiotics described in literature in broilers (Table 1) on the intestinal bacteria and colonisation resistance against different enteropathogens especially Salmonella spp., Campylobacter spp. and Clostridium spp. Overviews from studies on the influence of dietary prebiotics on the caecal microbial population (Table 2) and on bacterial fermentation metabolites in different segments of the intestine (Table 3) are also given.
Mannan-oligosaccharides
Mannan-oligosacharides (MOS), a glucomannoprotein complex derived from the cell wall of the yeasts, provides a mannose rich source for the attachment of some bacteria that would otherwise attach to the gut wall. The competition between MOS and the gut wall for the binding of bacteria involves the presence of lectins in the bacterial surface that are specific for some sugars. Lectins specific for mannose predominate in the intestinal pathogens (Escherichia coli, Salmonella enteritidis, Salmonella typhi, Salmonella typhimurium etc). These lectins, associated with pili or fimbriae of bacteria (Type-1 fimbriae) attach to mannose-containing cells in the intestinal tract (Oyofo et al., Reference OYOFO, DELOACH, CORRIER, NORMAN, ZIPRIN and MOLLENHAUER1989a; Reference OYOFO, DROLESKEY, NORMAN, MOLLENHAUER, ZIPRIN, CORRIER and DELOACH1989b; Spring et al., Reference SPRING, WENK, DAWSON K.A. and NEWMAN2000). MOS can be hydrolyzed by different species of Lactobacillus and some Bifidobacterium, however, it seems to be less fermentable by the intestinal bacteria than fructo-oligosaccharides (Zentek et al., Reference ZENTEK, MARQUART and PIETRZAK2002). It has been found that dietary inclusion of MOS enhanced the growth of caecal Lactobacillus spp. and Bifidobacterium spp., while numbers of coliforms and Clostridium perfringens were reduced in broilers (Shao et al., Reference SHAO, ZHOU, LI and LIN2000; Denev et al., Reference DENEV, NIKIFOROV and DINEV2005; Baurhoo et al., Reference BAURHOO, PHILLIP and RUIZ-FERIA2007).
On the other hand, dietary MOS failed to alter the caecal concentrations of anaerobes, lactic acid bacteria, coliforms, lactose-negative enterobacteria and Clostridium perfringens except that a greater population of coliforms has been found in the duodenal and ileal digesta of birds fed a diet supplemented with MOS compared to control birds (Yang et al., Reference YANG, IJI, KOCHER, MIKKELSEN and CHOCT2007). In laying hens, dietary MOS reduced the caecal counts of enterobacteria with a concomitant increase in Enterococcus spp. without affecting anaerobes, Clostridium spp., Lactobacillus spp., Eubacterium spp., coliforms, Fusobacterium spp. and Bacteroides spp. (Fernandez et al., Reference FERNANDEZ, HINTON and VAN GILS2002). Spring et al. (Reference SPRING, WENK, DAWSON K.A. and NEWMAN2000) could not find any difference in the caecal population of anaerobes, coliforms, Lactobacillus spp. and Enterococcus spp. in salmonella-infected broilers supplemented with MOS. The authors suggested that this lack of effect of MOS on Lactobacillus spp. and Enterococcus spp. was due to the absence of type-1 fimbriae on the bacterial surface. In the same study, MOS failed to alter the caecal concentrations of short chain fatty acids (SCFA) and lactate. However, in one study, caecal lactate was higher in MOS-fed broilers experimentally infected with Campylobacter jejuni and Campylobacter coli (Anderson et al., Reference ANDERSON, STANLEY, GRACE, TAYLOR, HUME and SEFTON A.E.2005). Dietary addition of MOS did not cause any major change in the SCFA profile of caecal digesta of broilers. However, the relative proportion of acetate was higher in MOS-fed broilers (Yang et al., Reference YANG, IJI, KOCHER, MIKKELSEN and CHOCT2007; Table 3) on day 14 but not on day 35.
MOS and colonisation resistance
There is a growing interest to investigate the specific and selective properties of some oligosaccharides and to describe how they can affect the colonisation of some pathogenic bacteria like Salmonella spp., Escherichia coli and Clostridium spp. in the intestine of broilers. Dietary MOS given to young chicks challenged with Salmonella typhimurium reduced caecal count of Salmonella typhimurium (Spring et al., Reference SPRING, WENK, DAWSON K.A. and NEWMAN2000). Similarly, in a second series of trials with Salmonella dublin as challenge organism, the number of young broilers that tested salmonella positive in the caeca was less when MOS was part of the diet. Fernandez et al. (Reference FERNANDEZ, HINTON and VAN GILS2000) studied the protection against the caecal colonisation of Salmonella enterica serovar Enteritidis in broilers when birds were inoculated with the caecal contents from hens fed a MOS-supplemented diet. Results showed that chicks were better protected with both a MOS-supplemented diet plus inoculation with the caecal culture obtained from hens that were also fed a MOS-supplemented diet. In another trial (Fernandez et al., Reference FERNANDEZ, HINTON and VAN GILS2002), the number of birds infected with Salmonella enterica serovar Enteritidis was lowered in MOS-fed birds compared to birds fed a control diet. MOS reduced the caecal count of Campylobacter spp. in broilers infected with ten different strains of Campylobacter jejuni and Campylobacter coli (Anderson et al., Reference ANDERSON, STANLEY, GRACE, TAYLOR, HUME and SEFTON A.E.2005). The effect of treatments was increased over the time as colonisation was reduced by 45.49% (or log 1.92) at 5 weeks. There is a paucity of information regarding the effect of MOS on Clostridium spp. in broilers. It has been reported that MOS effectively reduced the mortality and sub-clinical effects of Clostridium perfringens infection in broilers (Hofacre et al., Reference HOFACRE, BEACORN, COLLETT and MATHIS G.2003).
Fructo-oligosaccharides
Although indigestible oligosaccharides composed of fructose and glucose units are often referred to as fructo-oligosaccharides (FOS), this term refers especially to short chains (~3-6 units) of fructose units bounded by a ß(2-1) linkage, that are attached to a terminal glucose unit. FOS has been shown to enhance the growth of Bifidobacterium spp. and Lactobacillus spp. but to inhibit the growth of Escherichia coli and Salmonella spp. in the intestine (Hidaka et al., Reference HIDAKA, EIDA, TAKIZAWA, TOKUNAGA and TASHIRO1986; Choi et al., Reference CHOI, NAMKRNG and PAIK1994; Roberfroid et al., Reference ROBERFROID, VANLOO and GIBSON1998; Fukata et al., Reference FUKATA, SASAI, MIYAMOTO and BABA1999). Lactobacillus lactis, Enterococcus faecium and Pedicoccus spp. grew in media with pure FOS as the energy source, but not the tested Salmonella serotypes. All Salmonella serotypes that have been tested utilised an impure form of FOS (Oyarzbal and Conner, Reference OYARZBAL and CONNER1995). In vitro competitive experiments demonstrated a non-significant difference in the concentrations of Clostridium spp. when grown in FOS-containing media, either co-cultured with Bifidobacterium spp. or alone (Kullen et al., Reference KULLEN, KHIL, BUSTA, GALLAHER and BRADY L.J.1998). Supplementation of fructan-rich Jerusalem artichoke or topinambur syrup, containing inulin (degree of polymerization (DP) > 10), FOS (DP 5-10), oligofructose (DP 2-4), saccharose, fructose, glucose, minerals and vitamins in broilers reduced the caecal total aerobes, Clostridium perfringens and Enterobacteriaceae with a concomitant increase in the population of Bdellovibrio bacteriovorus (Kleessen et al., Reference KLEESSEN, ELSAYED, LOEHREN, SCHROEDL and KRUEGER M.2003; Table 2). A number of studies showed that dietary FOS influenced the intestinal bacterial community by increasing the population of Bifidobacterium spp., Lactobacillus spp. and Eubacterium spp. while decreasing the concentrations of Clostridium spp. and Escherichia coli (Wang and Shan, Reference WANG and SHAN2002; Cao et al., Reference CAO, ZHANG, GUO, YUAN and WIE2003; Xu et al., Reference XU, HU, XIA, ZHAN and WANG2003; Zhan et al., Reference ZHAN, HU and XU2003; Cao et al., Reference CAO, KARASAWA and GUO2005; Table 2). On the other hand, it has been shown that dietary FOS did not alter the caecal populations of Bifidobacterium spp., Bacteroides spp., Lactobacillus spp. and Escherichia coli in broilers (Fukata et al., Reference FUKATA, SASAI, MIYAMOTO and BABA1999; Yusrizal and Chen, Reference YUSRIZAL and CHEN2003). However, numbers of Lactobacillus spp. were elevated only in female broilers, without altering the numbers of Escherichia coli and total aerobes in the small intestine (Yusrizal and Chen, Reference YUSRIZAL and CHEN2003). Adding oligofructose to a diet for broilers reduced the faecal volatile ammonia at four weeks of age, but later concentrations returned to normal (Yusrizal and Chen, Reference YUSRIZAL and CHEN2003). Supplementation of FOS in broiler diets has been reported to decrease the concentrations of caecal phenols and indole without affecting total SCFA (Cao et al., Reference CAO, KARASAWA and GUO2005; Table 3) and to enhance the production of caecal SCFA (Zhan et al., Reference ZHAN, HU and XU2003) by elevating the concentration of butyrate (Zhan et al., Reference ZHAN, HU and XU2003) and valerate (Cao et al., Reference CAO, KARASAWA and GUO2005).
FOS and colonisation resistance
Attempts to control the colonisation of Salmonella spp. with dietary FOS have shown mixed results. Feeding FOS to broilers had little influence on the caecal colonisation of Salmonella spp. When FOS was fed in combination with competitive exclusion (CE) intestinal bacteria, there was a reduction in both number of salmonella positive-birds and caecal population of Salmonella spp. (Bailey et al., Reference BAILEY, BLANKENSHIP and COX1991). FOS has been found to reduce the population and number of salmonella positive-birds when broilers were subjected to feed withdrawal (Bailey et al., Reference BAILEY, BLANKENSHIP and COX1991), although this effect was not evident in broilers fed with FOS plus CE bacteria (Oyarzbal and Conner, Reference OYARZBAL and CONNER1996). It has been shown that there was a reduction in the caecal colonisation of Salmonella spp. in broilers fed either FOS alone (Choi et al., Reference CHOI, NAMKRNG and PAIK1994; Chamber et al., 1997; Fukata et al., Reference FUKATA, SASAI, MIYAMOTO and BABA1999) or in combination with CE bacteria (Fukata et al., Reference FUKATA, SASAI, MIYAMOTO and BABA1999).
Inulin
Inulin, extracted from chicory (Cichorium intybus), is a heterogeneous compound with respect to polymer chain length, containing molecules ranging from DP 3 to 60. The sugars moieties and the glycosidic linkages are similar to those of FOS, but with an average DP of 10. Inulin is believed to enhance the growth of health-promoting bacteria and suppresses the growth of potential pathogenic bacteria including Clostridium perfringens (Zentek et al., Reference ZENTEK, MARQUART, PIETRZAK, BALLEVRE and ROCHAT2003). In vitro fermentability of oligofructose and inulin was compared with a range of reference carbohydrates by measuring bacterial end-product formation in a batch culture. SCFA and gas formation indicated that these substrates were utilised by mixed populations of intestinal bacteria. The results showed that inulin exerted a preferential stimulatory effect on the genus Bifidobacterium, whilst maintaining populations of potential pathogens (Escherichia coli, Clostridium spp.) at relatively low levels (Wang and Gibson, Reference WANG and GIBSON1993).
In broilers, dietary inulin failed to alter the intestinal bacterial community, either determined by traditional culture-based technique (Biggs and Parsons, Reference BIGGS and PARSONS2005) technique or a culture-independent molecular fingerprinting technique (Rehman et al., Reference REHMAN, HELLWEG, TARAS and ZENTEK2008b). Using traditional culture methods, it has been reported that dietary inulin did not affect the numbers of total aerobes and Lactobacillus spp. (Yusrizal and Chen, Reference YUSRIZAL and CHEN2003) or Clostridium perfringens (H. Rehman, unpublished data) except for lowering concentrations of Escherichia coli/coliforms in the small intestine and caecal digesta of female, but not male, broilers (Yusrizal and Chen, Reference YUSRIZAL and CHEN2003). The reason for this difference is not clear and needs further consideration.
It seems that response of broilers to prebiotics may also depend upon gender, as it has been reported that bacterial community of the jejunum of broilers is affected by gender (Lumpkins et al., Reference LUMPKINS, BATAL and LEE2008). An increased concentration of caecal Bifidobacterium spp. has been found in inulin-fed one-week-old laying hens (Rada et al., Reference RADA, DUSKOVA, MAROUNEK, KOPECNY, SIMUNEK, AVGUSTIN and JAVORSKY2001). It has been shown that inulin did not clearly affect the colonisation of Salmonella spp. and Campylobacter spp. in the digesta of the small intestine and caeca, except for lower caecal salmonella count in female broilers (Yusrizal and Chen, Reference YUSRIZAL and CHEN2003).
Dietary inulin reduced faecal pH and volatile ammonia levels in broilers during the second week of age, although later it returned to normal (Yusrizal and Chen, Reference YUSRIZAL and CHEN2003). Rehman et al. (Reference REHMAN, BÖHM and ZENTEK2008a; Reference REHMAN, HELLWEG, TARAS and ZENTEK2008b) studied the intestinal microbial fermentation spectrum in inulin-fed broilers. Lactate was higher in the jejunal digesta of inulin-supplemented birds, but this effect could not be observed in the crop and gizzard digesta. Inulin did not affect the total concentrations of caecal SCFA; however, the proportion of butyrate was higher in supplemented birds (Table 3). Caecal ammonia was decreased in supplemented-birds without affecting the jejunal ammonia. Total biogenic amines in the jejunal and caecal digesta remained unaffected by the dietary supplementation of inulin, but putrescine was elevated in the caecal and jejunal digesta of inulin-supplemented birds compared to the control group.
Isomalto-oligosaccharides
Isomalto-oligosaccharide (IMO), produced from glucose by enzymatic transgalactosylation (Hayashi et al., Reference HAYASHI, HONITANI and IMADA1994; Vetere et al., Reference VETERE, GAMINI, CAMPA and PAOLETTI S.2000), is a mixture containing isomaltose, panose, isomaltotriose and several branched oligosaccharides composed of four or five glucose moieties. It can also be produced through the use of a modified Leuconostoc mesenteroides fermentation (Chung and Day, Reference CHUNG and DAY2002). IMO can be hydrolyzed by enzymes in the brush border of the small intestinal mucosal cells (Hunziker et al., Reference HUNZIKER, SPIESS, SEMENZA and LODISH1986; Hertel et al., Reference HERTEL, HEINZ and VOGEL2000). Despite this, it has been tested as a prebiotic similar to other non-digestible oligosaccharides in broilers (Xia et al., Reference XIA, XIE, QUAN and ZHOU2001; Zhang, Reference ZHANG2000; Zhang et al., Reference ZHANG, LI, LU and YI2003; Chung and Day, Reference CHUNG and DAY2004). Results from a pure culture technique indicated that IMO stimulated the growth of Bifidobacterium bifidum, Bifidobacterium longum and Lactobacillus johnsonii but not Salmonella typhimurium and Escherichia coli (Chung and Day, Reference CHUNG and DAY2002). Similarly, using IMO as the only carbohydrate source, data from competitive strain studies showed a higher viable count and faster growth of caecal isolates of chickens compared to salmonella populations when IMO was provided to caecal bacteria as a sole carbon source (Chung and Day, Reference CHUNG and DAY2004).
There are a few reports describing the influence of IMO on the intestinal bacteria in broilers. No differences were found in the population of total aerobes, Lactobacillus spp. or Escherichia coli in the digesta of crop and caecum of IMO-supplemented broilers (Zhang et al., Reference ZHANG, LI, LU and YI2003; Table 2). On the other hand, IMO selectively increased the caecal Bifidobacterium spp., but not total lactic acid bacteria or total anaerobic bacteria in broilers experimentally infected with Salmonella typhimurium (Thitaram et al., 2005; Table 2). Feeding IMO (1%) reduced the caecal population of Salmonella typhimurium, but this effect was lost when the supplementation level was increased to 2% or 4% (Thitaram et al., 2005). This example indicates that the dosage of prebiotic carbohydrates needs to be carefully adjusted to achieve the desired effects. The explanation is not well founded, but there might be negative interference with the digestive processes, perhaps due to a depression of ileal nutrient digestibility and subsequent higher substrate availability for pathogenic bacteria.
The addition of IMO at different dosages in a diet of broilers did not influence the concentrations of ileal and caecal SCFA; however, the feeding of 0.3% IMO elevated of the concentrations of n-butyrate and iso-butyrate in the jejunal digesta (Zhang et al., Reference ZHANG, LI, LU and YI2003).
Kestoses
Manley-Harris and Richards (Reference MANLEY-HARRIS and RICHARDS1991) produced a series of kestose oligosaccharides by pyrolysis of sucrose containing 1-kestose, 6-kestose and neo-kestose and their alpha fructofuranosyl anomers (iso-1-kestose, iso-6-kestose and iso-neokestose). Thermal treatment of amorphous anhydrous acidified sucrose produces sucrose thermal oligosaccharide caramel (STOC), a complex mixture containing fructose rich oligosaccharides and di-fructose di-anhydrides (Manley-Harris and Richards, Reference MANLEY-HARRIS and RICHARDS1994). Pure culture of Escherichia coli, Bacteroides thetaiotaomicron, Bifidobacterium bifidum and Clostridium perfringens did not grow on the thermal kestoses, although it stimulated the growth of Bacteroides fragilis, Bifidobacterium breve, Klebsiella pneumoniae and Clostridium butyricum. There was a slight growth of Lactobacillus acidophilus, Lactobacillus salivarius and Enterococcus faecalis (Patterson et al., Reference PATTERSON, ORBAN, SUTTON A.L. and RICHARDS1997). Inclusion of thermal kestoses in the diet of broilers increased the caecal population of Bifidobacterium spp. and Lactobacillus spp. without affecting the concentrations of total anaerobes and Clostridium spp. (Patterson et al., 1972; Table 2). Orban et al. (Reference ORBAN, PATTERSON, SUTTON and RICHARDS G.N.1997) studied the effects of different levels of STOC, dietary vitamin-mineral levels and brooding temperatures on the caecal bacterial population in broilers and received inconsistent results.
Lactose
Lactose can not be considered as a “true” prebiotic by definition, as enterocytes contain lactase. The activity of lactase is low in poultry (Siddons and Coates, Reference SIDDONS and COATES1972), with about 50% of ingested lactose in a poultry diet reaching the caeca (Atkinson et al., Reference ATKINSON, KRATZER and STEWART1957). Here it is fermented to lactate, as higher concentration of caecal lactate have been found in lactose-fed broilers (van der Wielen et al., Reference VAN DER WIELEN, VAN KNAPEN and BIESTERVELD2002) compared to low levels found in commercially raised broiler chickens fed a lactose-free diet (Rehman et al., Reference REHMAN, VAHJEN, AWAD and ZENTEK2007; Reference REHMAN, BÖHM and ZENTEK2008a). Lactose is believed to promote the growth of lactose-fermenting bacteria that either compete with Salmonella spp. colonisation or produce substances toxic to Salmonella spp. (DeLoach et al., Reference DELOACH, OYOFU, CORRIER, KUBENA, ZIPRIN and NORMAN1990). Chicks receiving dietary lactose showed larger numbers of Enterobacteriaceae and Enterococcus spp. on day 7 (van der Wielen et al., Reference VAN DER WIELEN, VAN KNAPEN and BIESTERVELD2002) or day 5 (Morishita et al., Reference MORISHITA, FULLER and COATES1982), while populations of Lactobacillus spp. remained unaffected in 2 week-old broilers (van der Wielen et al., Reference VAN DER WIELEN, VAN KNAPEN and BIESTERVELD2002). On the other hand, supplementation of lactose (2.5% or 4.5%) in the diets of broilers failed to affect the intestinal microbial population of Enterococcus spp., Lactobacillus spp., Clostridium spp. or Escherichia coli (McReynolds et al., Reference MCREYNOLDS, BYRD, GENOVESE, POOLE, DUKE, FARNELL and NISBET2007).
lactose and colonisation resistance
Dietary lactose, supplied either in drinking water or in feed, has been found to reduce the caecal colonisation of Salmonella spp. in broilers (Corrier et al., Reference CORRIER, HINTON, ZIPRIN and DELOACH1990a and b; Hinton et al., Reference HINTON, CORRIER, NORMAN, BEIER and DELOACH J.R.1990 and 1991; Nisbet et al., Reference NISBET, CORRIER, SCANLAN, HOLLISTER, BEIER and DELOACH J.R.1993) (Table 4). However, the protection against Salmonella spp. colonisation was greater in broilers when lactose was fed either in combination with an anaerobic caecal bacterial culture from adult broilers (Hinton et al., Reference HINTON, CORRIER, NORMAN, BEIER and DELOACH J.R.1990 and 1991; Nisbet et al., Reference NISBET, CORRIER, SCANLAN, HOLLISTER, BEIER and DELOACH J.R.1993 and 1994) or with Lactobacillus acidophilus (Qin et al., Reference QIN, FUKUTA, BABA and ARAKAWA1995). Nevertheless, lactose supplementation was unable to eliminate Salmonella spp. completely from birds. The anti-salmonella effect of dietary lactose may be attributed, in part, to an increased acidity of the digesta resulting from lactose fermentation. In salmonella-infected broilers, dietary lactose changed the intestinal bacterial fermentation profile characterised by a higher concentration of lactate and propionate and lower concentration of butyrate (Corrier et al., Reference CORRIER, HINTON, ZIPRIN and DELOACH1990a and b; Hinton et al., Reference HINTON, CORRIER, NORMAN, BEIER and DELOACH J.R.1990 and 1991). Waldroup et al. (Reference WALDROUP, YAMAGUCHI, SKINNER and WALDROUP1992) suggested that supplementing poultry diets with lactose (at 7.5%) was not a viable means of reducing or eliminating the incidence of Salmonella spp. or levels on processed broiler carcasses.
Clostridium spp. were found less frequently in the caecal digesta of lactose-fed broilers (Morishita et al., Reference MORISHITA, FULLER and COATES1982). It has been demonstrated that dietary lactose either reduced the number and caecal colonisation of Clostridium perfringens (Takeda et al., Reference TAKEDA, FUKATA, MIYAMOTO, SASAI, BABA and ARAKAWA1995) or the intestinal lesions associated with necrotic enteritis (McReynolds et al., Reference MCREYNOLDS, BYRD, GENOVESE, POOLE, DUKE, FARNELL and NISBET2007) in broilers experimentally infected with Clostridium perfringens.
Apart from other prebiotics tested in broilers, a few reports are also available for lactosucrose and lactulose. Lactosucrose, an oligosaccharide composed of galactose, glucose and fructose, is produced from lactose and sucrose by ß-fructofuranosidase from Arthrobacter spp. (Fujita et al., Reference FUJITA, HARA, SAKAI, HASHIMOTO and KITAHATA1990). In vitro studies showed that lactosucrose was fermented by all species of Bifidobacterium spp. tested except Bifidobacterium bifidum, but not by many strains of putrefactive bacteria like Clostridium perfringens and Enterobacteriaceae (Fujita et al., Reference FUJITA, HARA, SAKAI, MIYAE, YAMASITA, TSUNETOMI and MITSUOKA1991; Hara et al., Reference HARA, LI, SASAKI, MARUYAMA, TERADA, OGATA, FUJITA, ISHIGAMI, HARA, FUJIMORI and MITSUOKA1994). Terada et al. (Reference TERADA, HARA, SAKAMOTO, SATO, TAKAGI, MITSUOKA, MINO, HARA, FUJIMORI and YAMADA1994) studied the effect of supplementation of lactosucrose (0.15%) on the caecal microbial populations in broilers. Caecal concentrations of ammonia, phenol and cresol were decreased on day 62, while acetate and butyrate were higher on day 62 of lactosucrose consumption. Dietary lactosucrose elevated the caecal count of Bifidobacterium spp. on day 62 (Table 2). On day 20, the caecal population and the frequency of occurrence of lecithinase-negative Clostridium spp. was decreased without affecting lecithinase-positive Clostridium spp. However, lactosucrose did not affect the caecal population of Enterobacteriaeceae and Lactobacillus spp. (Table 2).
Lactulose, an isomeric form of lactose composed of galactose and glucose (Zentek et al., Reference ZENTEK, MARQUART and PIETRZAK2002), has also been tested for alleviating the undesirable egg taints in layers (Zentek and Kamphues, Reference ZENTEK and KAMPHUES2002). In vitro trials demonstrated that lactulose was fermented by Bifidobacterium spp. (Rycroft et al., Reference RYCROFT, JONES, GIBSON and RASTALL2001). Dietary lactulose and lactosucrose in broiler feed proved to be ineffective in eliminating Salmonella spp. (Chambers et al., Reference CHAMBERS, SPENCER and MODLER1997).
Conclusions
There is a lack of reported clinical trials demonstrating the role of prebiotics on the intestinal bacteria and their potential impact on the colonisation resistance. For a better understanding and development of prebiotics, sound knowledge about the composition and activity of the intestinal bacteria and interaction between bacteria and host is a prerequisite. Use of molecular methods certainly may help in defining the microbial diversity and its response following the use of prebiotics. The dietary intervention with prebiotics tested in broilers and the change in the numbers of Lactobacillus spp. and Bifidobacterium spp. is still doubtful, as not all the prebiotics tested in broilers selectively enriched these two desirable bacterial populations. Likewise, the desirable ability of prebiotics to reduce the numbers of putrefactive and potentially pathogenic organisms seems to be questionable depending upon the data presented for broilers. The observed variation may be due to the compositions of feed, structure of the feed, the age of birds and, most importantly, the doses and types of prebiotics studied. As prebiotics vary in their molecular structure and chain length, it is unlikely that they are fermented at equal rates or in the same part of the intestine resulting in a variable intestinal bacterial fermentation spectrum. Apart from classical prebiotics, research for some other potential candidates is also needed to evaluate how they may affect the microbial ecology of the anterior GIT, especially the crop and gizzard that acts as gateway for the entrance of different enteropathogens. Therefore, it might be interesting to develop a prebiotic or a mixture of differentially fermentable carbohydrates that contains both rapidly fermentable carbohydrates (acting in the upper digestive tract) and more slowly fermentable carbohydrates that may influence the microbial community of the lower intestinal tract.
