Tetracycline antibiotics transfer from contaminated milk to dairy products and the effect of the skimming step and pasteurisation process on residue concentrations

Pseudo-Siamese network combined with label-free Raman spectroscopy for the quantification of mixed trace amounts of antibiotics in human milk: A feasibility study

The utilization of antibiotics is prevalent among lactating mothers. Hence, the rapid determination of trace amounts of antibiotics in human milk is crucial for ensuring the healthy development of infants. In this study, we constructed a human milk system containing residual doxycycline (DXC) and/or tetracycline (TC). Machine learning models and clustering algorithms were applied to classify and predict deficient concentrations of single and mixed antibiotics via label-free SERS spectra. The experimental results demonstrate that the CNN model has a recognition accuracy of 98.85% across optimal hyperparameter combinations. Furthermore, we employed Independent Component Analysis (ICA) and the pseudo-Siamese Convolutional Neural Network (pSCNN) to quantify the ratios of individual antibiotics in mixed human milk samples. Integrating the SERS technique with machine learning algorithms shows significant potential for rapid discrimination and precise quantification of single and mixed antibiotics at deficient concentrations in human milk.

Cross-sectional, commercial testing, and chromatographic study of the occurrence of antibiotic residues throughout an artisanal raw milk cheese production chain

This study investigated antibiotic utilization in artisanal dairies and residue occurrence throughout the raw milk cheese production chain using commercial testing (Charm KIS and Eclipse Farm3G) and UHPLC-QqQ-MS/MS and LC-QqQ-MS/MS. The cross-sectional survey results revealed gaps in the producers' knowledge of antibiotic use. Commercial testing detected antibiotic levels close to the LOD in 12.5 % of the samples, mainly in raw milk and whey, with 10.0 % testing positive, specifically in fresh and ripened cheeses, indicating that antibiotics are concentrated during cheese-making. Chromatographically, several antibiotics were identified in the faeces of healthy animals, with chlortetracycline (15.7 ± 34.5 µg/kg) and sulfamethazine (7.69 ± 16.5 µg/kg) predominating. However, only tylosin was identified in raw milk (3.28 ± 7.44 µg/kg) and whey (2.91 ± 6.55 µg/kg), and none were found in fresh or ripened cheeses. The discrepancy between commercial and analytical approaches is attributed to compounds or metabolites not covered chromatographically.

Screening of antibiotic residues in raw bovine milk in Lombardy, Italy: Microbial growth inhibition assay and LC-HRMS technique integration for an accurate monitoring

Antibiotic residues in food of animal origin is a great concern for public health worldwide in terms of antibiotic resistance development, potential allergic reactions and disruption of intestinal flora equilibrium. In this study the presence of antibiotic residues in raw bovine milk samples collected from farms located in Lombardy region in Italy from 2018 to 2022 was assessed in the context of the national milk quality payment system. Samples were screened with microbiological growth inhibition test Delvotest ® SP NT and a very low positivity rate ranging from 0.1% to 0.07% over the four years was determined. A total of 79 positive samples were further analysed by LC-HRMS screening technique to confirm positivity and detect the specific antibiotic compound contaminating the sample. The β-lactam antibiotics resulted to be the most frequently detected, with the penicillin G being the most abundant compound. The data suggested that low levels of antibiotic contamination are consistently maintained over the last four years and the integration of the techniques used in this study is a valuable tool for a deep and precise monitoring of antibiotic residues in milk.

Toxicity of single and combined 4-epianhydrotetracycline and cadmium at environmentally relevant concentrations on the zebrafish embryos (Danio rerio)

The combined pollution of antibiotics and heavy metals has attracted a worldwide attention in the recent years. 4-epianhydrotetracycline (EATC) is the major degradation product of tetracycline (TC), which has been detected frequently in environment and its concentration is even higher than TC under some circumstances. Cadmium (Cd) is a common heavy metal contaminant and has highly toxic to organisms, plants and humans even at low doses. In the present study, zebrafish (Danio rerio) embryo toxicity test was performed to investigate the single and combined effects of EATC and Cd on aquatic organisms. Exposure to EATC and Cd at environmentally relevant concentrations had a series of hazardous impacts on the embryonic development, including lethality, hatching rate, heart rate and teratogenic effects. Compared to the contaminant existed alone, combined pollution produced stronger toxicity, which appeared as the decreasing of heart rate and hatching rate, and the increasing of malformation of zebrafish embryos. After 96 h exposure, the reactive oxygen species (ROS) levels in zebrafish embryos were increased significantly, revealing that EATC-Cd co-exposure resulted in potential oxidative stress-induced damage. Acridine orange (AO) staining showed that combined exposure resulted in stronger cell apoptosis. The potential health risks of the combined pollution of EATC and Cd should be paid more attention to higher level vertebrates and humans.

Tetracyclines in pasteurized milk commercialized in São Paulo, Brazil: occurrence and dietary risk assessment based on single and combined exposure with polyether ionophores

Tetracyclines and polyether ionophores are veterinary drugs frequently used in animals reared for food production. Despite the benefits, residues can be harmful for human health if levels are higher than those considered safe. The aim of this study was to evaluate, by LC-MS/MS, tetracyclines residues in pasteurised milk samples marketed in São Paulo state, Brazil and estimate the exposure of the adult/adolescent/children population. Within the eight antibiotics monitored two were detected, tetracycline and 4-epitetracycline, found in 22.5% and 12.5% of the samples, respectively. Since the same samples had been previously tested for polyether ionophore residues, risk assessment based on dietary exposure to tetracyclines when considered individually and in combination with the ionophores was performed. The daily intakes were below the acceptable daily intake (ADI) for tetracycline. Considering the levels of monensin reported in a previous study, the combined exposure with tetracycline and 4-epi-tetracycline also indicated low potential health concerns.

Transfer of antibiotics from goat's milk to rennet curd and whey fractions during cheese-making

The transfer of 35 antibiotics from milk to curd and whey was evaluated. Cheeses were produced at laboratory scale, from antibiotic-free goat's milk spiked with different antibiotic concentrations between 0.25 and 4 times the Maximum Residue Limits established in milk. Drug concentrations in milk, curd and whey were analysed by UHPLC-HRMS. Results indicated that most antibiotics were mainly transferred from milk to whey (up to 85.9%), with retention percentages in the curd lower than 50%, except for ceftiofur (59.7%) and dicloxacillin (52.8%). In most cases, drug distribution was unaffected by the antibiotic concentration in milk and correlated significantly to the drug lipophilicity (Log P) for β-lactams (R² = 0.54) and sulfonamides (R² = 0.62). When drug ionization was considered (Log D), improved correlation coefficients were obtained for macrolides (R² = 0.98). However, other factors besides the drug solubility should be considered to explain and predict the partitioning of antibiotics during cheese-making.

Antibiotics in Dairy Production: Where Is the Problem?

Antibiotics have long been used for the prevention and treatment of common diseases and for prophylactic purposes in dairy animals. However, in recent decades it has become a matter of concern due to the widespread belief that there has been an abuse or misuse of these drugs in animals and that this misuse has led to the presence of residues in derived foods, such as milk and dairy products. Therefore, this review aims to compile the scientific literature published to date on the presence of antibiotic residues in these products worldwide. The focus is on the reasons that lead to their presence in food, on the potential problems caused by residues in the characteristics of dairy products and in their manufacturing process, on the development and spread of antibiotic-resistant bacteria, and on the effects that both residues and resistant bacteria can cause on human and environmental health.

The effect of high hydrostatic pressure on tetracycline hydrochloride and sulfathiazole residues in various food matrices - comparison with ultrasound and heat treatment

Antibiotic residues in food pose serious direct and indirect risks for consumers. The aim of this study was to investigate the effect of High Hydrostatic Pressure (HHP) on tetracycline hydrochloride (TCH) and sulfathiazole (STZ) residues in honey, milk, and water. Three different pressures were tested for their efficiency and treatment at 580 MPa for 6 min was finally selected. Qualitative and quantitative determination of antibiotics were performed with HPLC and LC-MS. HHP treatment was compared to ultrasound and heat treatment. HHP treatment was found to be more effective than the other two methods for both antibiotics in water and milk. The reduction of STZ in honey was over 90%, while no reduction was observed for TCH. The highest TCH reduction was recorded after HHP treatment in water (76.4%) and the highest STZ reduction after ultrasound treatment in honey (94.3%). Reduction of the two antibiotics in different matrices did not follow a similar pattern. For the HHP treatment, the effect of the initial concentration of the two antibiotics was studied under two different storage conditions (refrigerated and frozen storage). The effectiveness of the method was found to be affected by the initial concentration, in both storage conditions for STZ, while for TCH significant differences were observed only for refrigerated storage.

Comparative Assessment of Antibiotic Residues Using Liquid Chromatography Coupled with Tandem Mass Spectrometry (LC-MS/MS) and a Rapid Screening Test in Raw Milk Collected from the North-Central Algerian Dairies

Antibiotic residues in milk are a major health threat for the consumer and a hazard to the dairy industry, causing significant economic losses. This study aims to assess the presence of antibiotic residues in raw milk comparatively by a rapid screening test (BetaStar^® Combo) and Liquid Chromatography coupled with Tandem Mass Spectrometry (LC-MS/MS). A total of 445 samples were collected from 3 dairy companies of north-central Algeria (Algiers, Blida, Boumerdes), and they were rapidly screened for β-lactams and tetracyclines; 52 samples, comprising 34 positive tanker-truck milk and 18 negative bulk-tank milk were tested by LC-MS/MS, which revealed 90.4% were contaminated (n = 47) and 55.3% exceeded the Maximum Residue Limit (MRL). The β-lactams as parent compounds and their metabolites were the most frequently detected with maximum value for cloxacillin (1231 µg/kg) and penicillin G (2062 µg/kg). Under field condition, the false-positive results, particularly for tetracyclines, seems to be related to milk samples displaying extreme acidity values (≥19°D) or fat-level fluctuations (2.7 g/100 mL and 5.6–6.2 g/100 mL). Despite a relatively low prevalence (7.64%) of residues using the rapid test, the detection by LC-MS/MS of flumequine (52 µg/kg), cefaclor (maximum 220 µg/kg) and metabolites of β-lactams at high levels should lead to reflections on the control of their human and environmental toxicological effects.

Transfer of certain beta-lactam antibiotics from cow's milk to fresh cheese and whey

The aim of this study was to investigate the transfer of cephalexin, penicillin-G, and ampicillin & cloxacillin from cow's milk to cheese and whey. For this purpose, raw milk was artificially contaminated to different antibiotic levels and then heat-treated to prepare fresh cheese from it. Antibiotic levels of the milk, whey and cheese were measured with LC-MS/MS. The extent of heat degradation was not sufficient to remove the antibiotic residues from milk. Antibiotic concentrations in whey and fresh cheese were in good accordance with the concentration of the same compound in milk suggesting that contamination of the milk will result in contamination of the product. The investigated antibiotics were transferred less into the cheese curd (1.6-12.5% of the original amount), than into the whey (33.2-74.1%). For penicillin-G even 100% (complete removal) was experienced.

Emerging Electrochemical Sensors for Real-Time Detection of Tetracyclines in Milk

Antimicrobial drug residues in food are strictly controlled and monitored by national laws in most territories. Tetracyclines are a major broad-spectrum antibiotic class, active against a wide range of Gram-positive and Gram-negative bacteria, and they are the leading choice for the treatment of many conditions in veterinary medicine in recent years. In dairy farms, milk from cows being treated with antibiotic drugs, such as tetracyclines, is considered unfit for human consumption. Contamination of the farm bulk tank with milk containing these residues presents a threat to confidence of supply and results in financial losses to farmers and dairy. Real-time monitoring of milk production for antimicrobial residues could reduce this risk and help to minimise the release of residues into the environment where they can cause reservoirs of antimicrobial resistance. In this article, we review the existing literature for the detection of tetracyclines in cow's milk. Firstly, the complex nature of the milk matrix is described, and the test strategies in commercial use are outlined. Following this, emerging biosensors in the low-cost biosensors field are contrasted against each other, focusing upon electrochemical biosensors. Existing commercial tests that identify antimicrobial residues within milk are largely limited to beta-lactam detection, or non-specific detection of microbial inhibition, with tests specific to tetracycline residues less prevalent. Herein, we review a number of emerging electrochemical biosensor detection strategies for tetracyclines, which have the potential to close this gap and address the industry challenges associated with existing tests.

Investigation of the effects of some processing conditions on the fate of oxytetracycline and tylosin antibiotics in the making of commonly consumed cheeses from the East Mediterranean

Background and Aim:

Transfer of antibiotics from raw milk to derived products is directly related to the processes involved in the manufacturing of dairy products, including East Mediterranean cheeses, since these have particular flow diagrams of production. The aim of this study was to assess the effects of skimming, pasteurization, curding, pressing, salting, cheese boiling, and whey acidification/heating on two widely used antibiotics in Lebanon, oxytetracycline (OTC) and tylosin (TYL), in the manufacture of commonly consumed cheeses in the East Mediterranean.

Materials and Methods:

Four hundred and fifty kilograms of full-fat bovine milk were spiked with OTC and TYL, then skimmed and pasteurized using holder and high-temperature short-time (HTST) methods. Milk was then processed to make cheeses (23 kg Baladi, 20 kg Akkawi, 20 kg Halloum, and 18 kg Double Cream). Liquid chromatography–mass-spectrometry was used to measure antibiotics. Analysis was performed using Statistical Package for the Social Sciences v25.

Results:

Skimming significantly (p=0.015) decreased TYL concentration by 68.6%. OTC degradation during holder (41-54%) proved to be significant (p=0.015). HTST had a significant (p=0.012) effect on TYL with 32% degradation. Curding step in making Baladi had a significant (p=0.028) effect on OTC only with the concentration increasing by 1.5-fold. Acidification and heating of whey to produce Double Cream decreased significantly (p=0.037) OTC concentration (14.7-46.3%), while TYL concentration increased significantly (p=0.000) by 300%. Pressing and salting in making Akkawi did not have any significant effect, while cheese boiling in making Halloum significantly decreased both antibiotics.

Conclusion:

OTC is transferred to Baladi and Akkawi (curd based) mainly, while double cream (whey based) has a high level of TYL transfer. Hence, people who consume these cheeses excessively could be exposed to high amounts of both antibiotics and thus be prone to their detrimental effect on health.

Multi-Class Procedure for Analysis of 50 Antibacterial Compounds in Eggshells Using Ultra-High-Performance Liquid Chromatography-Tandem Mass Spectrometry

In this work, for the first time, Ultra-High-Performance Liquid Chromatography–Tandem Mass Spectrometry (UHPLC–MS/MS) method was developed for qualitative and quantitative analysis of veterinary antibiotics (cephalosporins, diaminopyrimidines, fluoro(quinolones), lincosamides, macrolides, penicillins, pleuromutilins, sulfonamides, tetracyclines, and sulfones) in hen eggshells. The sample preparation method is based on a liquid–liquid extraction with a mixture of metaphosphoric acid, ascorbic acid, EDTA disodium salt dihydrate, and acetonitrile. The chromatographic separation was performed on Luna^® Omega Polar C18 10 column in gradient elution mode and quantitated in an 8 min run. Validation such as linearity, selectivity, precision, recovery, matrix effect, limit of quantification (LOQ), and limit of detection (LOD) was found to be within the acceptance criteria of the validation guidelines of the Commission Decision 2002/657/EC and EUR 28099 EN. Average recoveries ranged from 81–120%. The calculated LOQ values ranged from 1 to 10 µg/kg, the LOD values ranged from 0.3 to 4.0 µg/kg, depending on analyte. The developed method has been successfully applied to the determination of antibacterial compounds in hen eggshell samples obtained from different sources. The results revealed that enrofloxacin, lincomycin, doxycycline, and oxytetracycline were detected in hen eggshell samples.

Effects of Low-Dose Antibiotics on Gut Immunity and Antibiotic Resistomes in Weaned Piglets

Widespread antibiotic use increases the risk of livestock acting as potential reservoirs of antimicrobial resistance genes (ARGs) that may be transferred to human and animal pathogens. Particularly, maternal-infant transmission of antibiotics via breastmilk represents a great concern regarding infant health. In this study, we investigated the effects of 4-week low-dose antibiotic (LDA) treatment on the host immunity and antibiotic resistomes in weaned piglets. Transcriptomic analyses of ileum tissues revealed that the affected genes were largely enriched in innate immunity-related pathways. Significantly reduced protein expression of inflammatory factors, i.e., IFN-γ, IL-6 were observed. In addition, analyses of antibiotic resistomes identified a total of 1,021 ARGs related to 39 classes of antibiotics. The samples exhibited highly individual-specific diversity and no significant difference in the structure and diversity of ARGs and mobile gene elements (MGE) after LDA exposure for both colon and ileum samples. Despite of that, there were significant changes in the abundance of two transferrable ARGs [Erm(T) and tcr3] related to the antibiotics administered, implying an increased risk of transferrable antibiotic resistance. There was a significant change in the abundance of one pathogenic species after LDA exposure in the colon samples and one in the ileum samples, but there were no significant differences in the matched ARGs. Collectively, our findings reveal considerable changes in intestinal immunity-related genes, but minimal effects on gut antibiotic resistomes (ARGs and MGEs) in weaned piglets after 4 weeks LDA exposure. Our study provides a foundation for evaluating the longer-term cumulative effects of LDA use, especially the effects of maternal–infant LDA transmission, on antibiotic resistance and risks to infant health.

Short communication: Volatile profile of matured Tronchón cheese affected by oxytetracycline in raw goat milk

The presence of antibiotics in milk destined for cheese production may affect the biological processes responsible for the formation of volatile compounds, leading to alterations in the characteristic cheese flavor expected by consumers. The aim of this study was to evaluate the effect of the presence of oxytetracycline in goat milk on the volatile profile of ripened cheeses. Traditional mature Tronchón cheeses were manufactured from raw goat milk spiked with different concentrations of oxytetracycline (50, 100, and 200 µg/kg). Cheese made from antibiotic-free goat milk was used as control. We analyzed the residual amounts of the antibiotic and the volatile profile of the experimental cheeses on a fortnightly basis during maturation using liquid chromatography tandem mass spectrometry and then solid-phase microextraction followed by gas chromatography-mass spectrometry. Our results suggested that oxytetracycline was widely transferred from milk to cheese: drug concentrations in the cheeses were 3.5 to 4.3 times higher than those in raw milk. Although the residual amounts of oxytetracycline significantly decreased during maturation (88.8 to 96.5%), variable amounts of residues remained in cheese matured for 60 d (<10 to 79 µg/kg). In general, the presence of oxytetracycline in goat milk did not affect the volatile profile of Tronchón cheeses; volatile profile was significantly modified by ripening time. Still, the presence of oxytetracycline residues in cheeses ripened for 60 d could be of great concern for public health.

Paper-Based SERS Platform for One-Step Screening of Tetracycline in Milk

Throughout the last decade, the expansion of food testing has been gradually moving towards ordinary high throughput screening methods performed on-site. The demand for point-of-care testing, able to distinguish molecular signatures with high accuracy, sensitivity and specificity has been significantly increasing. This new requirement relies on the on-site detection and monitorization of molecular signatures suitable for the surveillance of food production and processing. The widespread use of antibiotics has contributed to disease control of livestock but has also created problems for the dairy industry and consumers. Its therapeutic and subtherapeutic use has increased the risk of contamination in milk in enough concentrations to cause economic losses to the dairy industry and have a health impact in highly sensitive individuals. This study focuses on the development of a simple Surface-Enhanced Raman Spectroscopy (SERS) method for fast high throughput screening of tetracycline (TET) in milk. For this, we integrate a paper-based low-cost, fully recyclable and highly stable SERS platform, with a minimal sample preparation protocol. A two-microliter sample of milk solutions spiked with TET (from 0.01 to 1000 ppm) is dried on a silver nanoparticle coated cardboard substrate and measured via a Raman spectrophotometer. The SERS substrate showed to be extremely stable with a shelf life of several months. A global spectrum principal component analysis approach was used to test all the detected vibrational modes and their correlation with TET concentration. Peak intensity ratios (455 cm-1/1280 cm-1 and 874 cm-1/1397 cm-1) were found to be correlated with TET concentrations in milk, achieving a sensitivity as low as 0.1 ppm. Results indicate that this SERS method combined with portable Raman spectrometer is a potential tool that can be used on-site for the monitoring of TET residues and other antibiotics.

Food Safety Margin Assessment of Antibiotics: Pasteurized Goat's Milk and Fresh Cheese

Traces of antimicrobials in milk are of great concern for public health. The European Union has established maximum residue limits in milk; these, however, by themselves do not guarantee the absence of drug residues in milk and related products. Currently, very little information is available on the transfer of antibiotic residues from milk to other dairy products and their potential effect on food safety. This work evaluated the presence of antibiotic residues in pasteurized fluid milk and fresh cheeses from goat's milk containing these veterinary drugs at legal safety levels (maximum residue limits) and assessed the safety margin of these dairy products for consumers. Eight antibiotics (amoxicillin, benzylpenicillin, cloxacillin, neomycin, erythromycin, ciprofloxacin, enrofloxacin, and oxytetracycline) were selected, and three batches of fresh cheese were made from pasteurized goat's milk spiked with each of these drugs. Drug residues in milk and cheese samples were analyzed by liquid chromatography-tandem mass spectrometry. The safety margin of goat's milk products was calculated taking into account different age groups (children, teenagers, and adults). Results showed that most antibiotics present in raw milk remained in pasteurized milk and were transferred to cheese to a high extent; retention was above 50% in most cases. The minimum safety margin in pasteurized milk was obtained for enrofloxacin, ciprofloxacin, and erythromycin for the children's group. For fresh cheese, an elevated safety margin was obtained for all antibiotics and age groups considered. However, the large amounts of antibiotics retained in the cheese might contribute to the development and spread of antimicrobial resistance. Considering the differences in milk from different species and the great variety of cheeses, it would be advisable to continue the traceability study of antibiotics in order to increase the safety margin of dairy products.

Characteristics of ripened Tronchón cheese from raw goat milk containing legally admissible amounts of antibiotics

The aim of this study was to evaluate the transfer of the most widely used antibiotics in dairy goats from milk to cheese as well as their effect on the cheese-making process and cheese characteristics during ripening. Antibiotic-free milk was spiked individually with 7 veterinary drugs (amoxicillin, benzylpenicillin, cloxacillin, erythromycin, ciprofloxacin, enrofloxacin, and oxytetracycline) at an equivalent concentration of the European Union maximum residue limit. Spiked goat milk was used to make mature Tronchón cheeses, which were analyzed at 0, 30, and 60 d of maturation to determine pH, chemical composition, proteolytic and lipolytic activities, and color and textural properties. A sensory evaluation of 60-d ripened cheeses was carried out. Cheeses from raw antibiotic-free goat milk were made simultaneously to be used as reference. The cheese-making process was unaffected by the presence of most antibiotics evaluated. Only erythromycin and oxytetracycline significantly increased the time required for cheese production (122 ± 29 and 108 ± 25 min, respectively). However, variable amounts of antibiotics, ranging from 7.4 to 68%, were transferred from milk to cheese, with oxytetracycline and quinolones showing the highest retention rates. In general, antibiotic residues present in the cheeses at the beginning of maturation decrease significantly along time. Thus, β-lactams and erythromycin residues were not detectable after 30 d of ripening. However, relatively high concentrations of enrofloxacin (148 ± 12 µg/kg) and ciprofloxacin (253 ± 24 µg/kg) residues were found in the cheeses after 60 d of maturation. The quality characteristics of the Tronchón cheeses were only slightly affected by such substances, with few significant differences in the free fatty acid concentration and color and textural properties of the cheeses. Results herein indicate that the use of goat milk containing antibiotics, such as quinolones, at the European Union maximum residue limit for cheese production could adversely affect the safety of the final products because relatively high concentrations of these substances could be retained in soft and semi-mature cheeses, making it necessary to assess the risk for consumer health. Studies on the partition of the antibiotic substances during cheese-making, using specific technologies, would be convenient to guarantee the safety of cheese and related products.

Chlortetracycline and melanin biopolymer - The risk of accumulation and implications for phototoxicity: An in vitro study on normal human melanocytes

Tetracyclines belong to antimicrobial classes with the highest consumption in veterinary medicine and agriculture, which leads to the contamination of the environment and food products, as well as to antibiotic resistance and adverse drug reactions. Chloro-derivatives of tetracyclines are thought to be relatively more phototoxic than others and belong to the most frequently cited drugs as photosensitizers. Melanins are heterogenous biopolymers determining skin, hair and eye colour. They are biosynthesized in a multistep process in melanocytes. Melanins, besides photoprotective and antioxidant properties, may also contribute to adverse skin drug reactions, which involve e.g. hyperpigmentation disorders and phototoxic reactions. Furthermore, they have the ability to form a drug-melanin complex, which leads to deposition of the drug or its metabolites in pigmented tissues. The aim of the study was to examine the ability of chlortetracycline to form a complex with melanin, as well as the effect of the drug on viability, antioxidant defence system and melanogenesis in normal human epidermal melanocytes exposed to the UVA radiation. The obtained results show for the first time that chlortetracycline forms a complex with melanin polymers, which creates a possibility of the drug accumulation in pigmented tissues. A simultaneous exposition of normal melanocytes to chlortetracycline and to the UVA radiation decreases cell viability, proportionally to the drug concentration and the irradiation time. The phototoxic effect appears to be related to the induction of oxidative stress in melanocytes, mainly through an increase of SOD and a decrease of the CAT activity. Chlortetracycline itself does not influence the melanin content or the activity of tyrosinase. The UVA radiation appeared to be a conditioning factor stimulating melanogenesis, whereas the presence of the drug augmented this effect.